THORNBURG INVESTMENT TRUST SUPPLEMENT

Dated September 30, 2024

to the

THORNBURG FUNDS PROSPECTUS

applicable to Class A, C, C2, D and I shares
dated February 1, 2024, as supplemented March 13, April 19, August 16, and August 21 2024,
(the “Retail Prospectus”)

CHANGES RELATING TO FUND BENCHMARKS

Effective September 30, 2024, as applicable, in the Fund Summary sections of the Retail Prospectus, the narrative language under the “Past Performance of the Fund” caption and/or the “Average Annual Total Returns” tables of each of the listed series of the Thornburg Investment Trust (the “Funds”) are hereby replaced with the disclosure below relating to changes to each Fund’s benchmark(s), which are being made in accordance with regulatory requirements and applicable to pages of the Retail Prospectus as follows:

The following information provides some indication of the risks of investing in the International Equity Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C and Class I share performance to the MSCI All Country World ex USA Net Total Return USD Index, a broad-based securities market index that represents the overall applicable market in which the Fund invests. The MSCI EAFE (Europe, Australasia, Far East) Net Total Return USD Index was removed as the Funds’ secondary index effective September 30, 2024. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

The following information provides some indication of the risks of investing in the International Growth Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares vary in each full year shown. The average annual total return figures compare Class A, Class C and Class I share performance to the MSCI All Country World ex USA Net Total Return, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the MSCI All Country World ex USA Growth Net Total Return USD Index, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Small/Mid Cap Core Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C and Class I share performance to the Russell 3000 Index – Total Return, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the Russell 2500 Index—Total Return, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The performance information shown above may include gains attributable to the Fund’s investments in shares of companies through initial public offerings (“IPOs”). There can be no assurance that the Fund will have continued access to profitable IPOs and, as the Fund’s assets grow, the impact of the Fund’s investment in IPOs on the performance of the Fund may decline.

The following information provides some indication of the risks of investing in the Small/Mid Cap Growth Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C and Class I share performance to the Russell 3000 Index – Total Return, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the Russell 2500 Index—Total Return, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Investment Income Builder Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares vary in each full year shown. The average annual total return figures compare Class A, Class C and Class I share performance to the MSCI World Net Total Return USD Index and the Bloomberg U.S. Aggregate Total Return Index Value USD, each a broad-based securities market index that represents the overall applicable market in which the Fund invests, and a Blended Benchmark, comprised of 25% Bloomberg U.S. Aggregate Bond Total Return Value USD, and 75% MSCI World Net Total Return USD Index, an additional benchmark that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Summit Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares of the Fund for each full year shown. The average annual total return figures compare Class A and Class I share performance to the MSCI All Country World Net Total Return USD Index and the Bloomberg Global-Aggregate Total Return Index Value Unhedged USD, each a broad-based securities market index that represents the overall applicable market in which the Fund invests, and a Blended Benchmark, comprised of 60% MSCI All Country World Index and 40% Bloomberg Global Aggregate Bond Index, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

The following information provides some indication of the risks of investing in the Ultra Short Income Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A and Class I share performance to the Bloomberg U.S. Aggregate Total Return Index Value USD, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the ICE BofA U.S. Treasury Bill Index, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation, and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Limited Term U.S. Government Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C, Class C2 and Class I share performance to the Bloomberg U.S. Aggregate Total Return Index Value USD, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the Bloomberg U.S. Government Intermediate Total Return Index Value Unhedged, an additional benchmark that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Limited Term Income Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C, Class C2 and Class I share performance to the Bloomberg U.S. Aggregate Total Return Index Value USD, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the Bloomberg Intermediate U.S. Government/Credit Total Return Index Value Unhedged, an additional benchmark that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Strategic Income Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows the annual total return for Class A shares in the one full year shown. The average annual total return figures compare Class A, Class C and Class I share performance to the Bloomberg U.S. Aggregate Total Return Index Value USD, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the Bloomberg U.S. Universal Total Return Index Value Unhedged, an additional benchmark that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. The Blended Benchmark, comprised of 80% Bloomberg U.S. Aggregate Bond Total Return Value USD and 20% MSCI World Net Total Return USD Index, was removed as the Fund’s secondary index effective September 30, 2024. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Short Duration Municipal Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A and Class I share performance to the ICE BofA U.S. Municipal Securities Index, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the ICE BofA 1-3 Year U.S. Municipal Securities Index, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation, and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Limited Term Municipal Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C, Class C2 and Class I share performance to the ICE BofA U.S. Municipal Securities Index, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the ICE BofA 1-10 Year U.S. Municipal Securities Index, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Intermediate Municipal Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C, Class C2 and Class I share performance to the ICE BofA U.S. Municipal Securities Index, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the ICE BofA 3-15 Year U.S. Municipal Securities Index, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund’s investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Strategic Municipal Income Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C and Class I share performance to the ICE BofA U.S. Municipal Securities Index, a broad-based securities market index that represents the overall applicable market in which the Fund invests. This index is not actively managed and is not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class A shares, and after-tax returns for other share classes will vary.

The following information provides some indication of the risks of investing in the Limited Term California Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C, Class C2 and Class I share performance to the ICE BofA U.S. Municipal Securities Index, a broad-based securities market index that represents the overall applicable market in which the Fund invests. The ICE BofA 1-10 Year U.S. Municipal Securities Index was removed as the Fund’s benchmark index effective September 30, 2024. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown is as of the calendar year ended December 31, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Average Annual Total Returns

(periods ended 12-31-23)