The Sustainable International Financial Institutions Act of 2025 aims to accelerate the global transition to a clean energy economy by significantly altering U.S. engagement with international financial institutions and its foreign assistance policies. It mandates that U.S. Executive Directors at specified institutions, including the World Bank and regional development banks, actively use their voice and vote to advance greenhouse gas reduction and channel assistance towards sustainable energy systems. A core provision requires these U.S. representatives to oppose any policy reform, investment, loan, or assistance that would create new or expand existing fossil fuel capacity, including refurbishment or life extension of such projects. The bill also supports phasing out funding for internal combustion engines in passenger vehicles and buses by 2027. Furthermore, it broadly defines "fossil fuel activity" to encompass exploration, production, transportation, combustion, and refining of coal, petroleum, and natural gas. To enforce these objectives, the Secretary of the Treasury must annually determine the amount IFIs spend on new fossil fuel capacity and reduce the U.S. contribution to that institution by an equivalent amount. These reduced funds are then deposited into an escrow account, only to be released once the Secretary certifies the institution is no longer supporting new fossil fuel activities. This mechanism provides a strong financial incentive for IFIs to align with clean energy goals. Finally, the legislation enacts a comprehensive prohibition on all direct or indirect U.S. government foreign assistance for any fossil fuel activity or related infrastructure project. This includes loans, insurance, guarantees, or technical assistance provided through agencies like the U.S. International Development Finance Corporation, the Export-Import Bank, and USAID, ensuring a unified U.S. stance against fossil fuel expansion globally.