THE RBB FUND, INC.

MFAM Global Opportunities Fund

Investor Shares: FOOLX

Institutional Shares: FOIIX

MFAM Mid-Cap Growth Fund

Investor Shares: TMFGX

Institutional Shares: FOGIX

Supplement dated April 30, 2020

to the Prospectus dated December 31, 2019

The following replaces the information in the Prospectus section entitled “Principal Risks – Investments in Common Stock – Market Risk”:

Market Risk. General market and economic factors may adversely affect securities markets generally and could, in turn, adversely affect the value of the Funds’ investments in common stocks, regardless of the performance or expected performance of companies in which the Funds invest.¹⁰ A Fund’s NAV and investment return will fluctuate based upon changes in the value of its investments. There is no assurance that a Fund will realize its investment objective, and an investment in a Fund is not, by itself, a complete or balanced investment program. You could lose money on your investment in a Fund, or a Fund could underperform other investments.

Periods of unusually high financial market volatility and restrictive credit conditions, at times limited to a particular sector or geographic area, have occurred in the past and may be expected to recur in the future. Some countries, including the United States, have adopted or have signaled protectionist trade measures, relaxation of the financial industry regulations that followed the financial crisis, and/or reductions to corporate taxes. The scope of these policy changes is still developing, but the equity and debt markets may react strongly to expectations of change, which could increase volatility, particularly if a resulting policy runs counter to the market’s expectations. The outcome of such changes cannot be foreseen at the present time. In addition, geopolitical and other risks, including environmental and public health risks, may add to instability in the world economy and markets generally. As a result of increasingly interconnected global economies and financial markets, the value and liquidity of a Fund’s investments may be negatively affected by events impacting a country or region, regardless of whether the Fund invests in issuers located in or with significant exposure to such country or region.

A recent outbreak of respiratory disease caused by a novel coronavirus was first detected in China in December 2019 and has spread internationally. The outbreak has resulted in closing borders and quarantines, enhanced health screenings, cancellations, disrupted supply chains and customer activity, and has produced general concern and uncertainty. The impact of this coronavirus, and other epidemics and pandemics that may arise in the future, could affect national and global economies, individual companies and the market in general in a manner that cannot be foreseen at the present time. Health crises caused by the recent outbreak may heighten other pre-existing political, social and economic risks in a country or region. In the event of a pandemic or an outbreak, there can be no assurance that the Funds and their service providers will be able to maintain normal business operations for an extended period of time or will not lose the services of key personnel on a temporary or long-term basis due to illness or other reasons. The full impacts of a pandemic or disease outbreaks are unknown, resulting in a high degree of uncertainty for potentially extended periods of time.

¹⁰ Stocks may go up and they may go down, sometimes quickly and unpredictably.

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Please retain this supplement for your reference.

THE RBB FUND, INC.

Motley Fool 100 Index ETF

(Cboe BZX: TMFC)

MFAM Small-Cap Growth ETF

(Cboe BZX: MFMS)

Supplement dated April 30, 2020

to the Prospectus dated December 31, 2019

The following replaces the information in the Prospectus section entitled “Additional Information About the Funds – Additional Principal Risk Information – Market Risk”:

Market Risk. The trading prices of equity securities and other instruments fluctuate in response to a variety of factors. A Fund’s NAV and market price is based upon the market’s perception of value and is not necessarily an objective measure of an investment’s value. There is no assurance that a Fund will realize its investment objective, and an investment in a Fund is not, by itself, a complete or balanced investment program. You could lose money on your investment in a Fund, or a Fund could underperform other investments.

Periods of unusually high financial market volatility and restrictive credit conditions, at times limited to a particular sector or geographic area, have occurred in the past and may be expected to recur in the future. Some countries, including the United States, have adopted or have signaled protectionist trade measures, relaxation of the financial industry regulations that followed the financial crisis, and/or reductions to corporate taxes. The scope of these policy changes is still developing, but the equity and debt markets may react strongly to expectations of change, which could increase volatility, particularly if a resulting policy runs counter to the market’s expectations. The outcome of such changes cannot be foreseen at the present time. In addition, geopolitical and other risks, including environmental and public health risks, may add to instability in the world economy and markets generally. As a result of increasingly interconnected global economies and financial markets, the value and liquidity of a Fund’s investments may be negatively affected by events impacting a country or region, regardless of whether the Fund invests in issuers located in or with significant exposure to such country or region.

A recent outbreak of respiratory disease caused by a novel coronavirus was first detected in China in December 2019 and has spread internationally. The outbreak has resulted in closing borders and quarantines, enhanced health screenings, cancellations, disrupted supply chains and customer activity, and has produced general concern and uncertainty. The impact of this coronavirus, and other epidemics and pandemics that may arise in the future, could affect national and global economies, individual companies and the market in general in a manner that cannot be foreseen at the present time. Health crises caused by the recent outbreak may heighten other pre-existing political, social and economic risks in a country or region. In the event of a pandemic or an outbreak, there can be no assurance that the Funds and their service providers will be able to maintain normal business operations for an extended period of time or will not lose the services of key personnel on a temporary or long-term basis due to illness or other reasons. The full impacts of a pandemic or disease outbreaks are unknown, resulting in a high degree of uncertainty for potentially extended periods of time.

* * * * *

Please retain this supplement for your reference.

THE RBB FUND, INC.

MFAM Global Opportunities Fund

Investor Shares: FOOLX

Institutional Shares: FOIIX

MFAM Mid-Cap Growth Fund

Investor Shares: TMFGX

Institutional Shares: FOGIX

Supplement dated April 30, 2020

to the Statement of Additional Information (“SAI”) dated December 31, 2019

The following supplements the information in the SAI section entitled “Investment Policies and Practices”:

Pandemic Risk

Disease outbreaks that affect local economies or the global economy may materially and adversely impact the Funds and/or the Adviser’s business. For example, uncertainties regarding the novel Coronavirus (“COVID-19”) outbreak have resulted in serious economic disruptions across the globe. These types of outbreaks can be expected to cause severe decreases in core business activities such as manufacturing, purchasing, tourism, business conferences and workplace participation, among others. These disruptions lead to instability in the market place, including stock market losses and overall volatility, as has occurred in connection with COVID-19. In the face of such instability, governments may take extreme and unpredictable measures to combat the spread of disease and mitigate the resulting market disruptions and losses. The Adviser has in place business continuity plans reasonably designed to ensure that it maintains normal business operations, and it periodically tests those plans. However, in the event of a pandemic or an outbreak, there can be no assurance that the Adviser or the Funds’ service providers will be able to maintain normal business operations for an extended period of time or will not lose the services of key personnel on a temporary or long-term basis due to illness or other reasons. The full impacts of a pandemic or disease outbreaks are unknown, resulting in a high degree of uncertainty for potentially extended periods of time.

* * * * *

Please retain this supplement for your reference.

THE RBB FUND, INC.

Motley Fool 100 Index ETF

(Cboe BZX: TMFC)

MFAM Small-Cap Growth ETF

(Cboe BZX: MFMS)

Supplement dated April 30, 2020

to the Statement of Additional Information (“SAI”) dated December 31, 2019

The following supplements the information in the SAI section entitled “Investment Policies and Practices”:

Pandemic Risk

Disease outbreaks that affect local economies or the global economy may materially and adversely impact the Funds and/or the Adviser’s business. For example, uncertainties regarding the novel Coronavirus (“COVID-19”) outbreak have resulted in serious economic disruptions across the globe. These types of outbreaks can be expected to cause severe decreases in core business activities such as manufacturing, purchasing, tourism, business conferences and workplace participation, among others. These disruptions lead to instability in the market place, including stock market losses and overall volatility, as has occurred in connection with COVID-19. In the face of such instability, governments may take extreme and unpredictable measures to combat the spread of disease and mitigate the resulting market disruptions and losses. The Adviser has in place business continuity plans reasonably designed to ensure that it maintains normal business operations, and it periodically tests those plans. However, in the event of a pandemic or an outbreak, there can be no assurance that the Adviser or the Funds’ service providers will be able to maintain normal business operations for an extended period of time or will not lose the services of key personnel on a temporary or long-term basis due to illness or other reasons. The full impacts of a pandemic or disease outbreaks are unknown, resulting in a high degree of uncertainty for potentially extended periods of time.

* * * * *

Please retain this supplement for your reference.