BLACKROCK SERIES FUND II, INC.

BlackRock High Yield Portfolio

(the “Fund”)

Supplement dated May 22, 2020 to the Prospectus dated May 1, 2020, as supplemented to date

Effective immediately, the following changes are made to the Fund’s Prospectus:

The first paragraph of the section of the Prospectus entitled “Fund Overview — Key Facts About BlackRock High Yield Portfolio — Principal Investment Strategies of the Fund” is deleted in its entirety and replaced with the following:

The Fund invests primarily in non-investment grade bonds with maturities of ten years or less. The Fund normally invests at least 80% of its assets in high yield bonds. The high yield securities (commonly called “junk bonds”) acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Fitch Ratings Inc. (“Fitch”), or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the Fund management team to be of similar quality. Split rated bonds and other fixed-income securities (securities that receive different ratings from two or more rating agencies) are valued as follows: if three agencies rate a security, the security will be considered to have the median credit rating; if two of the three agencies rate a security, the security will be considered to have the lower credit rating. The Fund may invest up to 30% of its assets in non-dollar denominated bonds of issuers located outside of the United States. The Fund’s investment in non-dollar denominated bonds may be on a currency hedged or unhedged basis. The Fund may also invest in convertible and preferred securities. These securities will be counted toward the Fund’s 80% policy to the extent they have characteristics similar to the securities included within that policy.

The first paragraph in the section of the Prospectus entitled “Details About the Fund — How the Fund Invests — Principal Investment Strategies” is deleted in its entirety and replaced with the following:

The Fund invests primarily in non-investment grade bonds with maturities of ten years or less. The Fund normally invests at least 80% of its assets in high yield bonds. The 80% policy is a non-fundamental policy of the Fund and may not be changed without 60 days’ prior notice to shareholders. The high yield securities (commonly called “junk bonds”) acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P or Fitch, or Ba or lower by Moody’s) or will be determined by the Fund management team to be of similar quality. Split rated bonds and other fixed-income securities (securities that receive different ratings from two or more rating agencies) are valued as follows: if three agencies rate a security, the security will be considered to have the median credit rating; if two of the three agencies rate a security, the security will be considered to have the lower credit rating. High yield securities are sometimes referred to as “junk bonds,” which are debt securities rated lower than investment grade (below the fourth highest rating of the major rating agencies). These securities generally pay more interest than higher rated securities. The higher yield is an incentive to investors who otherwise may be hesitant to purchase the debt of such a low-rated issuer. The Fund may invest up to 30% of its assets in non-dollar denominated bonds of issuers located outside of the United States. The Fund’s investment in non-dollar denominated bonds may be on a currency hedged or unhedged basis. The Fund may also invest in convertible and preferred securities. Convertible debt securities will be counted toward the Fund’s 80% policy to the extent they have characteristics similar to the securities included within that policy. Convertible securities generally are debt securities or preferred stock that may be converted into common stock. Convertible securities typically pay current income as either interest (debt security convertibles) or dividends (preferred stock convertibles). A convertible security’s value usually reflects both the stream of current income payments and the market value of the underlying stock. Preferred stock is a class of stock that often pays dividends at a specified rate and has preference over common stock in dividend payments and liquidation of assets. To add additional diversification, the management team can invest in a wide range of securities including corporate bonds, mezzanine investments, collateralized bond obligations, bank loans and mortgage-backed and asset-backed securities. The Fund can also invest, to the extent consistent with its investment objective, in non-U.S. and emerging market securities and currencies. The Fund may invest in securities of any rating, and

may invest up to 10% of its assets (measured at the time of investment) in distressed securities that are in default or the issuers of which are in bankruptcy. Mezzanine investments are subordinated debt securities that receive payments of interest and principal after other more senior security holders are paid. They are generally issued in private placements in connection with an equity security. Collateralized bond obligations are securities backed by a diversified pool of high yield securities. Bank loans are fixed and floating rate loans arranged through private negotiations between a company or a non-U.S. government and one or more financial institutions. The Fund considers such investments to be debt securities. Mortgage-backed securities are asset-backed securities based on a particular type of asset, a mortgage. There is a wide variety of mortgage-backed securities involving commercial or residential, fixed rate or adjustable rate mortgages and mortgages issued by banks or government agencies. Asset-backed securities are bonds that are backed by a pool of assets, usually loans such as installment sale contracts or credit card receivables.

Shareholders should retain this Supplement for future reference.

PRO-HY-0520SUP

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