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PIMCO FLEXIBLE MUNICIPAL INCOME FUND

(the "Fund")

Supplement dated April 7, 2020 to the Fund's Prospectus and Statement of Additional Information dated

December 24, 2019, each as supplemented from time to time

(respectively, the "Prospectus" and the "SAI")

Effective immediately, the first four paragraphs of the "Principal Risks of the Fund – Market Risk" section of the Prospectus Summary are deleted and replaced with the following:

Market Risk

The market price of securities owned by the Fund may go up or down, sometimes rapidly or unpredictably. Securities may decline in value due to factors affecting securities markets generally or particular industries represented in the securities markets. The value of a security may decline due to general market conditions which are not specifically related to a particular company, such as real or perceived adverse economic conditions, changes in the general outlook for corporate earnings, changes in interest or currency rates, adverse changes to credit markets or adverse investor sentiment generally. The value of a security may also decline due to factors which affect a particular industry or industries, such as labor shortages or increased production costs and competitive conditions within an industry. During a general downturn in the securities markets, multiple asset classes may decline in value simultaneously. Equity securities generally have greater price volatility than fixed income securities. Credit ratings downgrades may also negatively affect securities held by the Fund. Even when markets perform well, there is no assurance that the investments held by the Fund will increase in value along with the broader market.

In addition, market risk includes the risk that geopolitical and other events will disrupt the economy on a national or global level. For instance, war, terrorism, market manipulation, government defaults, government shutdowns, political changes or diplomatic developments, public health emergencies (such as the spread of infectious diseases, pandemics and epidemics) and natural/environmental disasters can all negatively impact the securities markets, which could cause the Fund to lose value. These events could reduce consumer demand or economic output, result in market closures, travel restrictions or quarantines, and significantly adversely impact the economy. The current contentious domestic political environment, as well as political and diplomatic events within the United States and abroad, such as the U.S. government's inability at times to agree on a long-term budget and deficit reduction plan, has in the past resulted, and may in the future result, in a government shutdown, which could have an adverse impact on the Fund's investments and operations. Additional and/or prolonged U.S. federal government shutdowns may affect investor and consumer confidence and may adversely impact financial markets and the broader economy, perhaps suddenly and to a significant degree. Governmental and quasi-governmental authorities and regulators throughout the world have previously responded to serious economic disruptions with a variety of significant fiscal and monetary policy changes, including but not limited to, direct capital infusions into companies, new monetary programs and dramatically lower interest rates. An unexpected or sudden reversal of these policies, or the ineffectiveness of these policies, could increase volatility in securities markets, which could adversely affect the Fund's investments. Any market disruptions could also prevent the Fund from executing advantageous investment decisions in a timely manner. To the extent that the Fund focuses its investments in a region enduring geopolitical market disruption, it will face higher risks of loss, although the increasing interconnectivity between global economies and financial markets can lead to events or conditions in one country, region or financial market adversely impacting a different country, region or financial market. Thus, investors should closely monitor current market conditions to determine whether the Fund meets their individual financial needs and tolerance for risk.

Current market conditions may pose heightened risks with respect to the Fund's investment in fixed income securities. As discussed more under "Principal Risks of the Fund – Interest Rate Risk," interest rates in the U.S. are at or near historically low levels. Any interest rate increases in the future could cause the value of the Fund to decrease. As such, fixed income securities markets may experience heightened levels of interest rate, volatility and liquidity risk.

Exchanges and securities markets may close early, close late or issue trading halts on specific securities or generally, which may result in, among other things, the Fund being unable to buy or sell certain securities or financial instruments at an advantageous time or accurately price its portfolio investments.

In addition, effective immediately, the first four paragraphs of the "Principal Risks of the Fund – Market Risk" section of the Prospectus are deleted and replaced with the following:

Market Risk

In addition, market risk includes the risk that geopolitical and other events will disrupt the economy on a national or global level. For instance, war, terrorism, market manipulation, government defaults, government shutdowns, political changes or diplomatic developments, public health emergencies (such as the spread of infectious diseases, pandemics and epidemics) and natural/environmental disasters can all negatively impact the securities markets, which could cause the Fund to lose value. These events could reduce consumer demand or economic output, result in market closures, travel restrictions or quarantines, and significantly adversely impact the economy. The current contentious domestic political environment, as well as political and diplomatic events within the United States and abroad, such as the U.S. government's inability at times to agree on a long-term budget and deficit reduction plan, has in the past resulted, and may in the future result, in a government shutdown, which could have an adverse impact on the Fund's investments and operations. Additional and/or prolonged U.S. federal government shutdowns may affect investor and consumer confidence and may adversely impact financial markets and the broader economy, perhaps suddenly and to a significant degree. Governmental and quasi-governmental authorities and regulators throughout the world have previously responded to serious economic disruptions with a variety of significant fiscal and monetary policy changes, including but not limited to, direct capital infusions into companies, new monetary programs and dramatically lower interest rates. An unexpected or sudden reversal of these policies, or the ineffectiveness of these policies, could increase volatility in securities markets, which could adversely affect the Fund's investments. Any market disruptions could also prevent the Fund from executing advantageous investment decisions in a timely manner. Funds that have focused their investments in a region enduring geopolitical market disruption will face higher risks of loss, although the increasing interconnectivity between global economies and financial markets can lead to events or conditions in one country, region or financial market adversely impacting a different country, region or financial market. Thus, investors should closely monitor current market conditions to determine whether the Fund meets their individual financial needs and tolerance for risk.

Effective immediately, the following is added after the "Principal Risks of the Fund – Operational Risk" section of the Prospectus Summary:

Market Disruptions Risk

as the spread of infectious diseases, pandemics and epidemics) and natural/environmental disasters, which can all negatively impact the securities markets and cause the Fund to lose value. These events can also impair the technology and other operational systems upon which the Fund's service providers, including PIMCO as the Fund's investment adviser, rely, and could otherwise disrupt the Fund's service providers' ability to fulfill their obligations to the Fund.

In addition, effective immediately, the following is added after the "Principal Risks of the Fund – Operational Risk" section of the Prospectus:

Market Disruptions Risk

The Fund is subject to investment and operational risks associated with financial, economic and other global market developments and disruptions, including those arising from war, terrorism, market manipulation, government interventions, defaults and shutdowns, political changes or diplomatic developments, public health emergencies (such as the spread of infectious diseases, pandemics and epidemics) and natural/environmental disasters, which can all negatively impact the securities markets and cause the Fund to lose value. These events can also impair the technology and other operational systems upon which the Fund's service providers, including PIMCO as the Fund's investment adviser, rely, and could otherwise disrupt the Fund's service providers' ability to fulfill their obligations to the Fund.

Effective immediately, the following is added after the first paragraph of the "Investment Objectives and Policies – Fund Operations" section of the SAI:

Market Disruptions Risk. The Fund is subject to investment and operational risks associated with financial, economic and other global market developments and disruptions, including those arising from war, terrorism, market manipulation, government interventions, defaults and shutdowns, political changes or diplomatic developments, public health emergencies (such as the spread of infectious diseases, pandemics and epidemics) and natural/environmental disasters, which can all negatively impact the securities markets and cause the Fund to lose value. These events can also impair the technology and other operational systems upon which the Fund's service providers, including PIMCO as the Fund's investment adviser, rely, and could otherwise disrupt the Fund's service providers' ability to fulfill their obligations to the Fund.

The recent spread of an infectious respiratory illness caused by a novel strain of coronavirus (known as COVID-19) has caused volatility, severe market dislocations and liquidity constraints in many markets, including markets for the securities the Fund holds, and may adversely affect the Fund's investments and operations. The transmission of COVID-19 and efforts to contain its spread have resulted in travel restrictions and disruptions, closed international borders, enhanced health screenings at ports of entry and elsewhere, disruption of and delays in healthcare service preparation and delivery, quarantines, event and service cancellations or interruptions, disruptions to business operations (including staff furloughs and reductions) and supply chains, and a reduction in consumer and business spending, as well as general concern and uncertainty that has negatively affected the economy. These disruptions have led to instability in the market place, including equity and debt market losses and overall volatility, and the jobs market. The impact of COVID-19, and other infectious illness outbreaks, epidemics or pandemics that may arise in the future, could adversely affect the economies of many nations or the entire global economy, the financial well-being and performance of individual issuers, borrowers and sectors and the health of the markets generally in potentially significant and unforeseen ways. In addition, the impact of infectious illnesses, such as COVID-19, in emerging market countries may be greater due to generally less established healthcare systems. This crisis or other public health crises may exacerbate other pre-existing political, social and economic risks in certain countries or globally.

The foregoing could lead to a significant economic downturn or recession, increased market volatility, a greater number of market closures, higher default rates and adverse effects on the values and liquidity of securities or other assets. Such impacts, which may vary across asset classes, may adversely affect the performance of the Fund. In certain cases, an exchange or market may close or issue trading halts on specific securities or even the entire market, which may result in the Fund being, among other things, unable to buy or sell certain securities or financial instruments or to accurately price their investments.

These and other developments may adversely affect the liquidity of the Funds' holdings (see "Liquidity Risk" in the Prospectus for further details).

Investors Should Retain This Supplement for Future Reference

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