UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED

MANAGEMENT INVESTMENT COMPANIES

Investment Company Act file number 811-22987

Pioneer ILS Interval Fund

(Exact name of registrant as specified in charter)

60 State Street, Boston, MA 02109

(Address of principal executive offices) (ZIP code)

Terrence J. Cullen, Amundi Asset Management, Inc.,

60 State Street, Boston, MA 02109

(Name and address of agent for service)

Registrant’s telephone number, including area code:  (617) 742-7825

Date of fiscal year end:  October 31, 2022

Date of reporting period: November 1, 2021 through April 30, 2022

Form N-CSR is to be used by management investment companies to file reports with the Commission not later than 10 days after the transmission to stockholders of any report that is required to be transmitted to stockholders under Rule 30e-1 under the Investment Company Act of 1940 (17 CFR 270.30e-1).  The Commission may use the information provided on Form N-CSR in its regulatory, disclosure review, inspection, and policymaking roles.

A registrant is required to disclose the information specified by Form N-CSR, and the Commission will make this information public. A registrant is not required to respond to the collection of information contained in Form N-CSR unless the Form displays a currently valid Office of Management and Budget (“OMB”) control number. Please direct comments concerning the accuracy of the information collection burden estimate and any suggestions for reducing the burden to Secretary, Securities and Exchange Commission, 450 Fifth Street, NW, Washington, DC 20549-0609.  The OMB has reviewed this collection of information under the clearance requirements of 44 U.S.C. ss. 3507.

Pioneer ILS
Interval Fund

Semiannual Report | April 30, 2022

Ticker Symbol: XILSX

visit us: www.amundi.com/us

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 1

President’s Letter

Dear Shareholders,

For two years now, investors have faced unprecedented challenges, as the COVID-19 pandemic has not only dominated the headlines since March 2020, but has also led to significant changes in government and central-bank policies, both in the US and abroad, and affected the everyday lives of each of us. With 2022 now well underway, the situation, while improved, has continued to evolve.

Widespread distribution of the COVID-19 vaccines approved for emergency use in late 2020 led to a general decline in virus-related hospitalizations in the US and had a positive effect on overall market sentiment during most of the 2021 calendar year. The passage of two additional fiscal stimulus packages by US lawmakers in December 2020 and January 2021 also helped drive a strong market rally. Then, the late-2021 emergence of the highly infectious Omicron variant of the virus led to surges in cases and hospitalizations, especially outside of the US, but also in certain areas of this country. That development contributed to a slowdown in the global economic recovery, as some foreign governments reinstated strict virus-containment measures that had been relaxed after the rollout of the vaccines. Many of those renewed restrictions were lifted as case numbers again began to decline during the late-winter months, but it appears the possibility of further virus-containment measures could be with us for a while longer, given that occasional surges in new cases have continued to arise, particularly in non-US locations.

In the US, while performance of most asset classes, especially equities, was positive for the full 2021 calendar year, 2022, so far, has featured a less-friendly market environment. Volatility in the fixed-income markets has remained high and we have seen negative returns for most asset classes. Meanwhile, equity markets, both domestic and global, have experienced significant underperformance over the first several months of the year. Concerns over global supply chain issues, rising inflation, the enactment of less-accommodative monetary policies from the Federal Reserve System (Fed), and partisan debates in Washington, DC over future spending and tax policies, are among the many factors that have led to greater uncertainty and an increase in market volatility. In addition, Russia's recent incursion into Ukraine has resulted in even greater market volatility, as economic sanctions placed on Russia by many Western countries have exacerbated the existing supply-chain issues and helped drive energy prices, including gas prices, to very high levels.

In our view, the long-term impact on the global economy from COVID-19, while currently unknown, is likely to be considerable, as it is clear that several industries have already felt greater effects than others, and could continue to struggle for quite some time. Of course, geopolitical concerns, whether they are related to the conflict in Ukraine or other crises in different areas of the globe, can always have an effect on the markets, and so our investment teams will remain vigilant and continue to monitor the geopolitical landscape.

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At the outset of the pandemic, we temporarily closed our offices and instituted a work-from-home policy, but have since re-opened our US locations. However, we have been maintaining all the necessary precautions, which at times may have us working more remotely than in person in order to ensure a safe working environment as new variants of the COVID-19 virus continue to arise and spread. I am proud of the careful planning that has taken place. Throughout the pandemic, our business has continued to operate without any disruption, and we all look forward to regaining a bit of normalcy after so many months of remote working.

Since 1928, Amundi US’s investment process has been built on a foundation of fundamental research and active management, principles which have guided our investment decisions for more than 90 years. We believe active management – that is, making active investment decisions – can help mitigate the risks during periods of market volatility.

At Amundi US, active management begins with our own fundamental, bottom-up research process. Our team of dedicated research analysts and portfolio managers analyzes each security under consideration, communicating frequently with the management teams of the companies and other entities issuing the securities, and working together to identify those securities that best meet our investment criteria for our family of funds. Our risk management approach begins with each and every security, as we strive to carefully understand the potential opportunity, while considering any and all risk factors.

Today, as investors, we have many options. It is our view that active management can serve shareholders well, not only when markets are thriving, but also during periods of market stress.

As you consider your long-term investment goals, we encourage you to work with your financial professional to develop an investment plan that paves the way for you to pursue both your short-term and long-term goals.

We greatly appreciate the trust you have placed in us and look forward to continuing to serve you in the future.

Sincerely,

Lisa M. Jones
Head of the Americas, President and CEO of US
Amundi Asset Management US, Inc.
June 2022

Any information in this shareowner report regarding market or economic trends or the factors influencing the Fund’s historical or future performance are statements of opinion as of the date of this report. Past performance is no guarantee of future results.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 3

Portfolio Management Discussion | 4/30/22

In the following interview, Chin Liu discusses the factors that affected the performance of the Pioneer ILS Interval Fund during the six-month period ended April 30, 2022. Mr. Liu, Managing Director, Director of Insurance-Linked Securities, Fixed-Income Solutions and Responsible Investment Research, and a portfolio manager at Amundi Asset Management US, Inc. (Amundi US), is responsible for the day-to-day management of the Fund, along with P. Campbell Brown, a vice president and a portfolio manager at Amundi US.

The first quarter of 2022 brought an increase in activity, with windstorms in Western and Central Europe, an earthquake in Japan, and floods on the east coast of Australia. According to the preliminary estimates, those events cost public and private insurers $14 billion. We feel it is important to note that the loss effects on the reinsurance industry have tended to be materially different than insured losses. All of the currently available information regarding those early-2022 events has been reflected in the Fund’s NAV as of period-end.

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The Fund posted losses in November and December 2021, but it registered gains in each of the first four months of 2022.

The Fund had exposure to the December 2021 Midwest storms, and to the early-2022 European winter storms and Australian floods mentioned earlier. Those exposures detracted from the Fund’s performance for the period.

The Fund’s positive returns from January through April were due mainly to limited natural catastrophes occurring during that timeframe. We believe the Fund’s solid performance during the last four months of the period helps illustrate the low degree of interest-rate sensitivity and the diversification potential that comes with investments in ILS, as ILS have been largely uncorrelated to the performance of the broader fixed-income market, which has experienced increased volatility and weak total returns over the first few months of the 2022 calendar year.

We have typically not invested the Fund in every new ILS deal; instead, we have continued to employ a comprehensive due diligence process to evaluate each investment for the appropriate structure as well as potential alignment of interest between the Fund and the ceding insurer. We have remained sponsor-agnostic, meaning that there are no direct affiliations or ownership conditions in place with a reinsurer. We believe that policy may help avoid potential conflicts of interest and could help limit the Fund’s exposures to the idiosyncratic risks associated with holding large positions with one reinsurer. We focus on the ILS market as

* Diversification does not assure a profit nor protect against loss.

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a whole, attempting to add value through our security selection process, by paying rigorous attention to sourcing investments that offer what we believe are attractive yields, and through careful management of the portfolio’s risk profile.

** Distributions/dividends are not guaranteed.

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During the six-month period, we sought to take advantage of the Fund’s potential capacity to benefit from market dislocations. The reinsurance industry has continued to shift away from the lower (riskier) layers and aggregate structures, due to concerns around event frequency. Our philosophy has historically been to avoid those types of layers and structures when choosing investments for the Fund. Instead, we have been working to position the portfolio in the less-risky area. We view our “agnostic” approach noted earlier as a critical factor at this point in the reinsurance underwriting cycle.

Many insurers, reinsurers, and observers have been expecting the “hard market” conditions to continue. A hard market is generally characterized by conditions when the price-per unit-of-risk has significantly increased. In our view, those conditions could represent a more favorable point in the reinsurance pricing cycle in which to deploy capital.

The majority of the calendar-year return stream in the ILS marketplace has tended to occur in the second half of the year, driven by seasonality. In years with limited losses, historically around 60% to 70% of the annual return for ILS has occurred in the second half.

Over the past several years, the investment industry has come to realize the importance and understanding of the potential effects of climate change. The environmental assessment of climate change and other weather-related risks is a complex undertaking and transcends nearly all types of capital-markets activities as well as investments. With regard to its relation to ILS, a wide range of stakeholders (insurers, reinsurers,

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 7

dedicated and non-dedicated ILS fund managers, and investors) have continued to translate and refine the potential effects of climate change into the asset class. The reinsurance industry has motivation to mitigate the costs associated with recovering from, and rebuilding after, low-probability, but high-severity events. For individuals, institutions, and governments, we believe the ability to have that type of capital available when needed can help build resilience across a wide range of communities. In order to deliver that potential benefit, we believe reinsurers must be at the forefront of the environmental aspects of climate change. In that regard, the industry has been using a data-driven approach of monitoring, modeling, and adjusting to changes in weather extremes as well as their potential effects on economies. Since the vast majority of ILS instruments are short term in nature, and prices are adjusted on an annual basis, factors such as frequency and severity of events can be used to develop a forward-looking “price” for that type of risk. The reinsurance industry, through its unique position in the market, may then provide price discovery as to the cost of weather risks potentially related to climate change.

More broadly speaking, we have continued to recognize the primary value proposition of ILS as an asset class with low structural correlation to other asset classes, and offering, in our view, a compelling set of risk/return characteristics. We believe those factors remain true as investors begin to confront the reality of rising interest rates in both the present and the near term, and possibly for longer.

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Please refer to the Schedule of Investments on pages 16–29 for a full listing of Fund securities.

All investments are subject to risk, including the possible loss of principal. In the past several years, financial markets have experienced increased volatility and heightened uncertainty. The market prices of securities may go up or down, sometimes rapidly or unpredictably, due to general market conditions, such as real or perceived adverse economic, political, or regulatory conditions, recessions, inflation, changes in interest or currency rates, lack of liquidity in the bond markets, the spread of infectious illness or other public health issues, armed conflict including Russia's military invasion of Ukraine, sanctions against Russia, other nations or individuals or companies and possible countermeasures, or adverse investor sentiment. These conditions may continue, recur, worsen or spread.

The Fund is a non-diversified, closed-end management investment company designed primarily as a long-term investment.

The Fund is not a complete investment program.

The Fund is operated as an interval fund, meaning the Fund will seek to conduct quarterly repurchase offers for a percentage of the Fund’s outstanding shares. Although the Fund will make quarterly repurchase offers, the Fund’s shares should be considered illiquid.

The Fund invests primarily in insurance-linked securities (“ILS”). ILS include event-linked bonds, quota share instruments (also known as “reinsurance sidecars”), collateralized reinsurance investments, industry loss warranties and other insurance and reinsurance-related securities.

The Fund could lose a portion or all of the principal it has invested in an ILS, and the right to additional interest and/or dividend payments with respect to the security, upon the occurrence of one or more trigger events, as defined within the terms of the security. Trigger events may include natural or other perils of a specific size or magnitude that occur in a designated geographic region during a specified time period, and/or that involve losses or other metrics that exceed a specific amount.

ILS may expose the Fund to other risks, including, but not limited to, issuer (credit) default, adverse regulatory or jurisdictional interpretations and adverse tax consequences. The Fund has limited transparency into the individual contracts underlying certain ILS, which may make the risk assessment of such securities more difficult.

The size of the ILS market may change over time, which may limit the availability of ILS for investment. The availability of ILS in the secondary market may also be limited.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 9

ILS in which the Fund invests may have limited liquidity or may be illiquid and, therefore, may be impossible or difficult to purchase, sell, or unwind. ILS also may be difficult to value.

The values of Fund holdings may go up or down, due to market conditions, inflation, changes in interest or currency rates, lack of liquidity in the financial markets or adverse investor sentiment.

Investments in high yield or lower-rated securities are subject to greater-than-average price volatility, illiquidity, and possibility of default.

When interest rates rise, the prices of fixed income securities held by the Fund will generally fall. Conversely, when interest rates fall, the prices of fixed income securities held by the Fund will generally rise.

The Fund’s investments, payment obligations and financing terms may be based on floating rates, such as LIBOR (London Interbank Offered Rate). Plans are underway to phase out the use of LIBOR. There remains uncertainty regarding the nature of any replacement rate and the impact of the transition from LIBOR on the Fund, issuers of instruments in which the Fund invests, and financial markets generally.

Investments in the Fund are subject to possible loss due to the financial failure of issuers of underlying securities and their inability to meet their debt obligations.

The Fund may use derivatives, such as swaps, inverse floating-rate obligations and others, which can be illiquid, may disproportionately increase losses, and have a potentially large impact on the Fund’s performance. Derivatives may have a leveraging effect.

Investing in foreign and/or emerging market securities involves risks relating to interest rates, currency exchange rates, and economic and political conditions.

To the extent the Fund invests a significant percentage of its assets in a single industry, such as the financial segment, the Fund may be particularly susceptible to adverse economic, regulatory or other events affecting that industry and may be more risky than a Fund that does not concentrate in an industry.

As a non-diversified Fund, the Fund can invest a higher percentage of its assets in the securities of any one or more issuers than a diversified fund. Being non-diversified may magnify the Fund’s losses from adverse events affecting a particular issuer.

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These risks may increase share price volatility.

Before investing, consider the product’s investment objectives, risks, charges and expenses. Contact your advisor or Amundi Asset Management US, Inc., for a prospectus or summary prospectus containing this information. Read it carefully. There is no assurance that these and other strategies used by the Fund will be successful.

Any information in this shareholder report regarding market or economic trends or the factors influencing the Fund’s historical or future performance are statements of opinion as of the date of this report. Past performance is no guarantee of future results.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 11

Portfolio Summary | 4/30/22

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Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 13

Prices and Distributions | 4/30/22

The data shown above represents past performance, which is no guarantee of future results.

Index Definition

The ICE Bank of America (ICE BofA) 3-Month U.S. Treasury Bill Index is an unmanaged market index of U.S. Treasury securities maturing in 90 days, that assumes reinvestment of all income. Index returns are calculated monthly, assume reinvestment of dividends and, unlike Fund returns, do not reflect any fees, expenses or sales charges. It is not possible to invest directly in an index.

The index defined here pertains to the “Value of $10,000 Investment” chart appearing on page 15.

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Performance Update | 4/30/22

Investment Returns

The mountain chart on the right shows the change in market value, plus reinvested dividends and distributions, of a $1,000,000 investment made in common shares of Pioneer ILS Interval Fund during the periods shown, compared to that of the ICE Bank of America (ICE BofA) 3-Month U.S. Treasury Bill Index.

Call 1-844-391-3034 or visit www.amundi.com/us for the most recent month-end performance results. Current performance may be lower or higher than the performance data quoted.

Performance data shown represents past performance. Past performance is no guarantee of future results. Investment return and market price will fluctuate, and shares, when redeemed, may be worth more or less than their original cost.

NAV results represent the percent change in net asset value per share. Performance, including short-term performance, is not indicative of future results. All results are historical and assume the reinvestment of dividends and capital gains.

The Fund has no sales charges. Performance results reflect any applicable expense waivers in effect during the periods shown. Without such waivers Fund performance would be lower. Waivers may not be in effect for all funds. Certain fee waivers are contractual through a specified period. Otherwise, fee waivers can be rescinded at any time. See the prospectus and financial statements for more information.

The performance table and graph do not reflect the deduction of fees and taxes that a shareowner would pay on Fund distributions.

Please refer to the financial highlights for a more current expense ratio.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 15

Schedule of Investments | 4/30/22 (unaudited)

The accompanying notes are an integral part of these financial statements.

16 Pioneer ILS Interval Fund | Semiannual Report | 4/30/22

The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 17

Schedule of Investments | 4/30/22

(unaudited) (continued)

The accompanying notes are an integral part of these financial statements.

18 Pioneer ILS Interval Fund | Semiannual Report | 4/30/22

The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 19

Schedule of Investments | 4/30/22

(unaudited) (continued)

The accompanying notes are an integral part of these financial statements.

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The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 21

Schedule of Investments | 4/30/22

(unaudited) (continued)

The accompanying notes are an integral part of these financial statements.

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The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 23

Schedule of Investments | 4/30/22

(unaudited) (continued)

The accompanying notes are an integral part of these financial statements.

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The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 25

Schedule of Investments | 4/30/22

(unaudited) (continued)

The accompanying notes are an integral part of these financial statements.

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Principal amounts are denominated in U.S. dollars (“USD”) unless otherwise noted.

EUR – Euro

Purchases and sales of securities (excluding short term investments) for the six months ended April 30, 2022, aggregated $230,641,534 and $313,616,515, respectively.

The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 27

Schedule of Investments | 4/30/22

(unaudited) (continued)

The Fund is permitted to engage in purchase and sale transactions (“cross trades”) with certain funds and accounts for which Amundi Asset Management US, Inc. (the “Adviser”) serves as the Fund’s investment adviser, as set forth in Rule 17a-7 under the Investment Company Act of 1940, pursuant to procedures adopted by the Board of Trustees. Under these procedures, cross trades are effected at current market prices. During the six months ended April 30, 2022, the Fund engaged in purchases of $793,379 and sales of $6,650,104 pursuant to these procedures, which resulted in a net realized gain/(loss) of $36,236.

At April 30, 2022, the net unrealized depreciation on investments based on cost for federal tax purposes of $929,404,233 was as follows:

Various inputs are used in determining the value of the Fund’s investments. These inputs are summarized in the three broad levels below.

Level 1 – unadjusted quoted prices in active markets for identical securities.

Level 2 – other significant observable inputs (including quoted prices for similar securities, interest rates, prepayment speeds, credit risks, etc.). See Notes to Financial Statements —Note 1A.

Level 3 – significant unobservable inputs (including the Fund’s own assumptions in determining fair value of investments). See Notes to Financial Statements — Note 1A.

The following is a summary of the inputs used as of April 30, 2022, in valuing the Fund’s investments:

The accompanying notes are an integral part of these financial statements.

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The following is a reconciliation of assets valued using significant unobservable inputs (Level 3):

The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 29

Statement of Assets and Liabilities | 4/30/22

(unaudited)

The accompanying notes are an integral part of these financial statements.

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The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 31

Statements of Changes in Net Assets

The accompanying notes are an integral part of these financial statements.

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The accompanying notes are an integral part of these financial statements.

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 33

Financial Highlights

The accompanying notes are an integral part of these financial statements.

34 Pioneer ILS Interval Fund | Semiannual Report | 4/30/22

Notes to Financial Statements | 4/30/22

(unaudited)

1. Organization and Significant Accounting Policies

Pioneer ILS Interval Fund (the “Fund”) was organized as a Delaware statutory trust on July 15, 2014. Prior to commencing operations on December 22, 2014, the Fund had no operations other than matters relating to its organization and registration as a non-diversified, closed-end management investment company under the Investment Company Act of 1940, as amended (the “1940 Act”). The investment objective of the Fund is to seek total return.

Amundi Asset Management US, Inc., an indirect wholly owned subsidiary of Amundi and Amundi’s wholly owned subsidiary, Amundi USA, Inc., serves as the Fund’s investment adviser (the “Adviser”). Amundi Distributor US, Inc., an affiliate of the Adviser, serves as the Fund’s distributor (the “Distributor”). Shares are offered at the Fund’s current net asset value (“NAV”) per share.

The Fund’s ability to accept offers to purchase shares may be limited when appropriate investments for the Fund are not available. Shares are generally available for purchase by registered investment advisers acting in a fiduciary capacity on behalf of their clients and by or through other qualified intermediaries and programs sponsored by such qualified financial intermediaries. Shares are also available to certain direct investors, which may be individuals, trusts, foundations and other institutional investors. Initial investments are subject to investment minimums described in the prospectus. Registered investment advisers and other financial intermediaries may impose different or additional minimum investment and eligibility requirements from those of the fund. The Adviser or the Distributor may waive the Fund’s minimum investment requirements.

The Fund is an “interval” fund and makes periodic offers to repurchase shares (See Note 7). Except as permitted by the Fund’s structure, no shareowner will have the right to require the Fund to repurchase its shares. No public market for shares exists, and none is expected to develop in the future. Consequently, shareowners generally will not be able to liquidate their investment other than as a result of repurchases of their shares by the Fund.

In March 2020, FASB issued an Accounting Standard Update, ASU 2020-04, Reference Rate Reform (Topic 848) — Facilitation of the Effects of Reference Rate Reform on Financial Reporting (“ASU 2020-04”), which provides

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optional, temporary relief with respect to the financial reporting of contracts subject to certain types of modifications due to the planned discontinuation of the London Interbank Offered Rate (“LIBOR”) and other LIBOR-based reference rates at the end of 2021. The temporary relief provided by ASU 2020-04 is effective for certain reference rate-related contract modifications that occur during the period from March 12, 2020 through December 31, 2022. Management is evaluating the impact of ASU 2020-04 on the Fund’s investments, derivatives, debt and other contracts, if applicable, that will undergo reference rate-related modifications as a result of the reference rate reform.

The Fund is an investment company and follows investment company accounting and reporting guidance under U.S. Generally Accepted Accounting Principles (“U.S. GAAP”). U.S. GAAP requires the management of the Fund to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of income, expenses and gain or loss on investments during the reporting period. Actual results could differ from those estimates.

The following is a summary of significant accounting policies followed by the Fund in the preparation of its financial statements:

The net asset value of the Fund is computed once daily, on each day the New York Stock Exchange (“NYSE”) is open, as of the close of regular trading on the NYSE.

Event-linked bonds are valued at the bid price obtained from an independent third party pricing service. Other insurance-linked securities (including reinsurance sidecars, collateralized reinsurance and industry loss warranties) may be valued at the bid price obtained from an independent pricing service, or through a third party using a pricing matrix, insurance industry valuation models, or other fair value methods or techniques to provide an estimated value of the instrument.

Fixed-income securities are valued by using prices supplied by independent pricing services, which consider such factors as market prices, market events, quotations from one or more brokers, Treasury spreads, yields, maturities and ratings, or may use a pricing matrix or other fair value methods or techniques to provide an estimated value of the security or instrument. A pricing matrix is a means of valuing a debt security on the basis of current market prices for other debt securities, historical trading patterns in the market for fixed-income securities and/or other factors. Non-U.S. debt securities that are listed on an exchange will be valued at

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the bid price obtained from an independent third party pricing service. When independent third party pricing services are unable to supply prices, or when prices or market quotations are considered to be unreliable, the value of that security may be determined using quotations from one or more broker-dealers.

The value of foreign securities is translated into U.S. dollars based on foreign currency exchange rate quotations supplied by a third party pricing source. Trading in non-U.S. equity securities is substantially completed each day at various times prior to the close of the NYSE. The values of such securities used in computing the net asset value of the Fund’s shares are determined as of such times. The Fund may use a fair value model developed by an independent pricing service to value non-U.S. equity securities.

Forward foreign currency exchange contracts are valued daily using the foreign exchange rate or, for longer term forward contract positions, the spot currency rate and the forward points on a daily basis, in each case provided by a third party pricing service. Contracts whose forward settlement date falls between two quoted days are valued by interpolation.

Shares of open-end registered investment companies (including money market mutual funds) are valued at such funds’ net asset value. Shares of exchange-listed closed-end funds are valued by using the last sale price on the principal exchange where they are traded.

Securities for which independent pricing services or broker-dealers are unable to supply prices or for which market prices and/or quotations are not readily available or are considered to be unreliable are valued by a fair valuation team comprised of certain personnel of the Adviser pursuant to procedures adopted by the Fund’s Board of Trustees. The Adviser’s fair valuation team uses fair value methods approved by the Valuation Committee of the Board of Trustees. The Adviser’s fair valuation team is responsible for monitoring developments that may impact fair valued securities and for discussing and assessing fair values on an ongoing basis, and at least quarterly, with the Valuation Committee of the Board of Trustees.

Inputs used when applying fair value methods to value a security may include credit ratings, the financial condition of the company, current market conditions and comparable securities. The Fund may use fair value methods if it is determined that a significant event has occurred after the close of the exchange or market on which the security trades and prior to the determination of the Fund’s net asset value. Examples of a significant event might include political or economic news, corporate restructurings,

Pioneer ILS Interval Fund | Semiannual Report | 4/30/22 37

natural disasters, terrorist activity or trading halts. Thus, the valuation of the Fund’s securities may differ significantly from exchange prices, and such differences could be material.

At April 30, 2022, one security was valued using fair value methods (in addition to securities valued using prices supplied by independent pricing services, broker-dealers or a third party insurance industry broker valuation model) representing 0.0% of net assets. The value of these fair valued securities was $0.

Dividend income is recorded on the ex-dividend date, except that certain dividends from foreign securities where the ex-dividend date may have passed are recorded as soon as the Fund becomes aware of the ex-dividend data in the exercise of reasonable diligence.

Interest income, including interest on income-bearing cash accounts, is recorded on the accrual basis. Dividend and interest income are reported net of unrecoverable foreign taxes withheld at the applicable country rates and net of income accrued on defaulted securities.

Interest and dividend income payable by delivery of additional shares is reclassified as PIK (payment-in-kind) income upon receipt and is included in interest and dividend income, respectively.

Security transactions are recorded as of trade date. Gains and losses on sales of investments are calculated on the identified cost method for both financial reporting and federal income tax purposes.

The books and records of the Fund are maintained in U.S. dollars. Amounts denominated in foreign currencies are translated into U.S. dollars using current exchange rates.

Net realized gains and losses on foreign currency transactions, if any, represent, among other things, the net realized gains and losses on foreign currency exchange contracts, disposition of foreign currencies and the difference between the amount of income accrued and the U.S. dollars actually received. Further, the effects of changes in foreign currency exchange rates on investments are not segregated on the Statement of Operations from the effects of changes in the market prices of those securities, but are included with the net realized and unrealized gain or loss on investments.

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It is the Fund’s policy to comply with the requirements of the Internal Revenue Code applicable to regulated investment companies and to distribute all of its net taxable income and net realized capital gains, if any, to its shareowners. Therefore, no provision for federal income taxes is required. As of October 31, 2021, the Fund did not accrue any interest or penalties with respect to uncertain tax positions, which, if applicable, would be recorded as an income tax expense on the Statement of Operations. Tax returns filed within the prior three years remain subject to examination by federal and state tax authorities.

The amount and character of income and capital gain distributions to shareowners are determined in accordance with federal income tax rules, which may differ from U.S. GAAP. Distributions in excess of net investment income or net realized gains are temporary over distributions for financial statement purposes resulting from differences in the recognition or classification of income or distributions for financial statement and tax purposes. Capital accounts within the financial statements are adjusted for permanent book/tax differences to reflect tax character, but are not adjusted for temporary differences.

The tax character of current year distributions payable will be determined at the end of the current taxable year. The tax character of distributions paid during the year ended October 31, 2021 was as follows:

The following shows the components of distributable earnings (losses) on a federal income tax basis at October 31, 2021:

The difference between book-basis and tax-basis net unrealized depreciation is attributable to the tax adjustments relating to wash sales, ILS securities, and mark to market of forward contracts.

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The value of securities held by the Fund may go up or down, sometimes rapidly or unpredictably, due to general market conditions, such as real or perceived adverse economic, political or regulatory conditions, recessions, the spread of infectious illness or other public health issues, inflation, changes in interest rates, armed conflict including Russia’s military invasion of Ukraine, sanctions against Russia, other nations or individuals or companies and possible countermeasures, lack of liquidity in the bond markets or adverse investor sentiment. In the past several years, financial markets have experienced increased volatility, depressed valuations, decreased liquidity and heightened uncertainty. These conditions may continue, recur, worsen or spread. Interest rates are very low, which means there is more risk that they may go up. The U.S. Federal Reserve has recently started to raise certain interest rates. A general rise in interest rates could adversely affect the price and liquidity of fixed-income securities. Rates of inflation have recently risen. The value of assets or income from an investment may be worth less in the future as inflation decreases the value of money. As inflation increases, the real value of the Fund’s assets can decline as can the value of the Fund’s distributions.

The global pandemic of the novel coronavirus respiratory disease designated COVID-19 has resulted in major disruption to economies and markets around the world, including the United States. Global financial markets have experienced extreme volatility and severe losses, and trading in many instruments has been disrupted. Liquidity for many instruments has been greatly reduced for periods of time. Some sectors of the economy and individual issuers have experienced particularly large losses. These circumstances may continue for an extended period of time, and may continue to affect adversely the value and liquidity of the Fund’s investments. Following Russia’s recent invasion of Ukraine, Russian securities have lost all, or nearly all, their market value. Other securities or markets could be similarly affected by past or future geopolitical or other events or conditions.

Governments and central banks, including the U.S. Federal Reserve, have taken extraordinary and unprecedented actions to support local and global economies and the financial markets. These actions have resulted in significant expansion of public debt, including in the U.S. The consequences of high public debt, including its future impact on the economy and securities markets, may not be known for some time.

The Fund invests primarily in insurance-linked securities (“ILS”). ILS may include event-linked bonds (also known as insurance-linked bonds or catastrophe bonds), quota share instruments (also known as “reinsurance sidecars”), collateralized reinsurance investments, industry loss warranties,

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event-linked swaps, securities of companies in the insurance or reinsurance industries, and other insurance and reinsurance-related securities. The Fund could lose a portion or all of the principal it has invested in an ILS, and the right to additional interest or dividend payments with respect to the security, upon the occurrence of one or more trigger events, as defined within the terms of an insurance-linked security. ILS carry significant risk. See note 1.G. At times, the Fund’s investments may represent industries or industry sectors that are interrelated or have common risks, making the Fund more susceptible to any economic, political, or regulatory developments or other risks affecting those industries and sectors.

The Fund’s investments in foreign markets and countries with limited developing markets may subject the Fund to a greater degree of risk than investments in a developed market. These risks include disruptive political or economic conditions, military conflicts and sanctions, terrorism, sustained economic downturns, financial instability, reduction of government or central bank support, inadequate accounting standards, tariffs, tax disputes or other tax burdens, nationalization or expropriation of assets, and the imposition of adverse governmental laws, arbitrary application of laws and regulations or lack of rule of law, or currency exchange restrictions.

Russia launched a large-scale invasion of Ukraine on February 24, 2022. In response to the military action by Russia, various countries, including the U.S., the United Kingdom, and European Union issued broad-ranging economic sanctions against Russia and Belarus and certain companies and individuals. Since then, Russian securities have lost all, or nearly all, their market value, and many other issuers, securities and markets have been adversely affected. The United States and other countries may impose sanctions on other countries, companies and individuals in light of Russia’s military invasion. The extent and duration of the military action or future escalation of such hostilities, the extent and impact of existing and future sanctions, market disruptions and volatility, and the result of any diplomatic negotiations cannot be predicted. These and any related events could have a significant impact on the value and liquidity of certain Fund investments, on Fund performance and the value of an investment in the Fund, particularly with respect to securities and commodities, such as oil and natural gas, as well as other sectors with exposure to Russian issuers or issuers in other countries affected by the invasion, and are likely to have collateral impacts on market sectors globally.

The Fund’s investments, payment obligations and financing terms may be based on floating rates, such as LIBOR (London Interbank Offered Rate). ICE Benchmark Administration, the administrator of LIBOR, ceased publication of most LIBOR settings on a representative basis at the end of

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2021 and is expected to cease publication of a majority of U.S. dollar LIBOR settings on a representative basis after June 30, 2023. In addition, global regulators have announced that, with limited exceptions, no new LIBOR-based contracts should be entered into after 2021. Actions by regulators have resulted in the establishment of alternative reference rates to LIBOR in most major currencies. Markets are developing in response to these new rates, but questions around liquidity in these rates and how to appropriately adjust these rates to eliminate any economic value transfer at the time of transition remain a significant concern. The effect of any changes to - or discontinuation of - LIBOR on the Fund will vary depending on, among other things, existing fallback provisions in individual contracts and whether, how, and when industry participants develop and widely adopt new reference rates and fallbacks for both legacy and new products and instruments. In March, 2022, the U.S. federal government enacted legislation to establish a process for replacing LIBOR in existing contracts that do not already provide for the use of a clearly defined or practicable replacement benchmark rate as described in the legislation. Generally speaking, for contracts that do not contain a fallback provision as described in the legislation, a benchmark replacement recommended by the Federal Reserve Board will effectively automatically replace the USD LIBOR benchmark in the contract after June 30, 2023. The recommended benchmark replacement will be based on the Secured Overnight Financing Rate (SOFR) published by the Federal Reserve Bank of New York, including any recommended spread adjustment and benchmark replacement conforming changes. The process of transitioning from LIBOR may involve, among other things, increased volatility or illiquidity in markets for instruments that rely on LIBOR. The transition may also result in a reduction in the value of certain LIBOR-based investments held by the Fund or reduce the effectiveness of related transactions such as hedges.

Any such effects of the transition away from LIBOR, as well as other unforeseen effects, could result in losses for the Fund. Because the usefulness of LIBOR as a benchmark may deteriorate during the transition period, these effects could occur at any time.

With the increased use of technologies such as the Internet to conduct business, the Fund is susceptible to operational, information security and related risks. While the Fund’s Adviser has established business continuity plans in the event of, and risk management systems to prevent, limit or mitigate, such cyber-attacks, there are inherent limitations in such plans and systems, including the possibility that certain risks have not been identified. Furthermore, the Fund cannot control the cybersecurity plans and systems put in place by service providers to the Fund such as the Fund’s custodian and accounting agent, and the Fund’s transfer agent. In addition, many beneficial owners of Fund shares hold them through

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accounts at broker-dealers, retirement platforms and other financial market participants over which neither the Fund nor the Adviser exercises control. Each of these may in turn rely on service providers to them, which are also subject to the risk of cyber-attacks. Cybersecurity failures or breaches at the Adviser or the Fund’s service providers or intermediaries have the ability to cause disruptions and impact business operations, potentially resulting in financial losses, interference with the Fund’s ability to calculate its net asset value, impediments to trading, the inability of Fund shareowners to effect share purchases or redemptions or receive distributions, loss of or unauthorized access to private shareowner information and violations of applicable privacy and other laws, regulatory fines, penalties, reputational damage, or additional compliance costs. Such costs and losses may not be covered under any insurance. In addition, maintaining vigilance against cyber-attacks may involve substantial costs over time, and system enhancements may themselves be subject to cyber-attacks.

The Fund’s prospectus contains unaudited information regarding the Fund’s principal risks. Please refer to that document when considering the Fund’s principal risks.

Restricted Securities are subject to legal or contractual restrictions on resale. Restricted securities generally are resold in transactions exempt from registration under the Securities Act of 1933. Private placement securities are generally considered to be restricted except for those securities traded between qualified institutional investors under the provisions of Rule 144A of the Securities Act of 1933.

Disposal of restricted investments may involve negotiations and expenses, and prompt sale at an acceptable price may be difficult to achieve. Restricted investments held by the Fund at April 30, 2022 are listed in the Schedule of Investments.

The Fund invests in ILS. The Fund could lose a portion or all of the principal it has invested in an ILS, and the right to additional interest or dividend payments with respect to the security, upon the occurrence of one or more trigger events, as defined within the terms of an insurance-linked security. Trigger events, generally, are hurricanes, earthquakes, or other natural events of a specific size or magnitude that occur in a designated geographic region during a specified time period, and/or that involve losses or other metrics that exceed a specific amount. There is no way to accurately predict whether a trigger event will occur, and accordingly, ILS carry significant

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risk. The Fund is entitled to receive principal, and interest and/or dividend payments so long as no trigger event occurs of the description and magnitude specified by the instrument. In addition to the specified trigger events, ILS may expose the Fund to other risks, including but not limited to issuer (credit) default, adverse regulatory or jurisdictional interpretations and adverse tax consequences.

The Fund’s investments in ILS may include event-linked bonds. ILS also may include special purpose vehicles (“SPVs”) or similar instruments structured to comprise a portion of a reinsurer’s catastrophe-oriented business, known as quota share instruments (sometimes referred to as reinsurance sidecars), or to provide reinsurance relating to specific risks to insurance or reinsurance companies through a collateralized instrument, known as collateralized reinsurance. Structured reinsurance investments also may include industry loss warranties (“ILWs”). A traditional ILW takes the form of a bilateral reinsurance contract, but there are also products that take the form of derivatives, collateralized structures, or exchange-traded instruments.

Where the ILS are based on the performance of underlying reinsurance contracts, the Fund has limited transparency into the individual underlying contracts, and therefore must rely upon the risk assessment and sound underwriting practices of the issuer. Accordingly, it may be more difficult for the Adviser to fully evaluate the underlying risk profile of the Fund’s structured reinsurance investments, and therefore the Fund’s assets are placed at greater risk of loss than if the Adviser had more complete information. Structured reinsurance instruments generally will be considered illiquid securities by the Fund. These securities may be difficult to purchase, sell or unwind. Illiquid securities also may be difficult to value. If the Fund is forced to sell an illiquid asset, the Fund may be forced to sell at a loss.

The Fund may enter into forward foreign currency exchange contracts (“contracts”) for the purchase or sale of a specific foreign currency at a fixed price on a future date. All contracts are marked-to-market daily at the applicable exchange rates, and any resulting unrealized appreciation or depreciation is recorded in the Fund’s financial statements. The Fund records realized gains and losses at the time a contract is offset by entry into a closing transaction or extinguished by delivery of the currency. Risks may arise upon entering into these contracts from the potential inability of counterparties to meet the terms of the contract and from unanticipated movements in the value of foreign currencies relative to the U.S. dollar (see Note 5).

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During the six months ended April 30, 2022, the Fund had entered into various forward foreign currency exchange contracts that obligated the Fund to deliver or take delivery of currencies at specified future maturity dates. Alternatively, prior to the settlement date of a forward foreign currency exchange contract, the Fund may close out such contract by entering into an offsetting contract.

The average market value of forward foreign currency exchange contracts open during the six months ended April 30, 2022, was Note- open for now and $0 for buys and sells, respectively. Open forward foreign currency exchange contracts outstanding at April 30, 2022, are listed in the Schedule of Investments.

Information on financial transactions which have been settled through the receipt or disbursement of cash or restricted cash is presented in the Statement of Cash Flows. Cash as presented in the Fund’s Statement of Assets and Liabilities includes cash on hand at the Fund’s custodian bank and does not include any short-term investments. As of and for the six months ended April 30, 2022, the Fund had no restricted cash presented on the Statement of Assets and Liabilities.

2. Management Agreement

The Adviser manages the Fund’s portfolio. Management fees payable under the Fund’s Investment Management Agreement with the Adviser are calculated daily and paid monthly at the annual rate of 1.75% of the Fund’s average daily net assets up to $1 billion, and 1.70% of the fund’s average daily net assets over $1 billion. For the six months ended April 30, 2022, the effective management fee was equivalent to 1.75% (annualized) of the Fund’s average daily net assets.

In addition, under the management and administration agreements, certain other services and costs, including accounting, regulatory reporting and insurance premiums, are paid by the Fund as administrative reimbursements. Included in “Due to affiliates” reflected on the Statement of Assets and Liabilities is $194,674 in management fees, administrative costs and certain other reimbursements payable to the Adviser at April 30, 2022.

3. Compensation of Trustees and Officers

The Fund pays an annual fee to its Trustees. The Adviser reimburses the Fund for fees paid to the Interested Trustees. The Fund does not pay any salary or other compensation to its officers. For the six months ended April 30, 2022, the Fund paid $20,669 in Trustees’ compensation, which is

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reflected on Statement of Operations as Trustees’ fees. At April 30, 2022, the Fund had a payable for Trustees’ fees on its Statement of Assets and Liabilities of $3,603.

4. Transfer Agent

For the period from November 1, 2021 to November 21, 2021, DST Systems, Inc. served as the transfer agent to the Fund at negotiated rates. Effective November 22, 2021, BNY Mellon Investment Servicing (US) Inc. serves as the transfer agent to the Fund at negotiated rates. Transfer agent fees and payables shown on the Statement of Operations and the Statement of Assets and Liabilities, respectively, include sub-transfer agent expenses incurred through the Fund’s omnibus relationship contracts.

In addition, the Fund reimbursed the transfer agent for out-of-pocket expenses incurred by the transfer agent related to shareowner communications activities such as proxy and statement mailings, and outgoing phone calls. For the six months ended April 30, 2022, such out-of-pocket expenses by class of shares were as follows:

5. Master Netting Agreements

The Fund has entered into an International Swaps and Derivatives Association, Inc. Master Agreement (“ISDA Master Agreement”) or similar agreement with substantially all of its derivative counterparties. An ISDA Master Agreement is a bilateral agreement between the Fund and a counterparty that governs the trading of certain Over the Counter (“OTC”) derivatives and typically contains, among other things, close-out and set-off provisions which apply upon the occurrence of an event of default and/or a termination event as defined under the relevant ISDA Master Agreement. The ISDA Master Agreement may also give a party the right to terminate all transactions traded under such agreement if, among other things, there is deterioration in the credit quality of the other party.

Upon an event of default or a termination of the ISDA Master Agreement, the non-defaulting party has the right to close-out all transactions under such agreement and to net amounts owed under each transaction to determine one net amount payable by one party to the other. The right to close out and net payments across all transactions under the ISDA Master Agreement could result in a reduction of the Fund’s credit risk to its counterparty equal to any amounts payable by the Fund under the

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applicable transactions, if any. However, the Fund’s right to set-off may be restricted or prohibited by the bankruptcy or insolvency laws of the particular jurisdiction to which each specific ISDA Master Agreement of each counterparty is subject.

The collateral requirements for derivatives transactions under an ISDA Master Agreement are governed by a credit support annex to the ISDA Master Agreement. Collateral requirements are generally determined at the close of business each day and are typically based on changes in market values for each transaction under an ISDA Master Agreement and netted into one amount for such agreement. Generally, the amount of collateral due from or to a counterparty is subject to threshold (a “minimum transfer amount”) before a transfer is required, which may vary by counterparty. Collateral pledged for the benefit of the Fund and/or counterparty is held in segregated accounts by the Fund’s custodian and cannot be sold, re-pledged, assigned or otherwise used while pledged. Cash that has been segregated to cover the Fund’s collateral obligations, if any, will be reported separately on the Statement of Assets and Liabilities as “Swaps collateral”. Securities pledged by the Fund as collateral, if any, are identified as such in the Schedule of Investments.

Financial instruments subject to an enforceable master netting agreement, such as an ISDA Master Agreement, have been offset on the Statement of Assets and Liabilities. The following charts show gross assets and liabilities of the Fund as of April 30, 2022.

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6. Additional Disclosures about Derivative Instruments and Hedging Activities

The Fund’s use of derivatives may enhance or mitigate the Fund’s exposure to the following risks:

Interest rate risk relates to the fluctuations in the value of interest-bearing securities due to changes in the prevailing levels of market interest rates.

Credit risk relates to the ability of the issuer of a financial instrument to make further principal or interest payments on an obligation or commitment that it has to the Fund.

Foreign exchange rate risk relates to fluctuations in the value of an asset or liability due to changes in currency exchange rates.

Equity risk relates to the fluctuations in the value of financial instruments as a result of changes in market prices (other than those arising from interest rate risk or foreign exchange rate risk), whether caused by factors specific to an individual investment, its issuer, or all factors affecting all instruments traded in a market or market segment.

Commodity risk relates to the risk that the value of a commodity or commodity index will fluctuate based on increases or decreases in the commodities market and factors specific to a particular industry or commodity.

The fair value of open derivative instruments (not considered to be hedging instruments for accounting disclosure purposes) by risk exposure at April 30, 2022, was as follows:

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The effect of derivative instruments (not considered to be hedging instruments for accounting disclosure purposes) on the Statement of Operations and Statement of Cash Flows by risk exposure at April 30, 2022, was as follows: