Sustainable International Financial Institutions Act of 2020 This bill establishes requirements to promote clean energy and climate justice globally through international financing mechanisms. Specifically, the bill requires the U.S. Executive Directors of specified international financial institutions to use the voice and vote of the United States to (1) advance the cause of reducing carbon emissions and transitioning the global economy to a clean energy economy, and (2) oppose any loan or extension of financial or technical assistance to a country or entity to create new capacity for fossil fuel activity. The Department of the Treasury must reduce the yearly U.S. contribution to a financial institution by the amount of any loans or assistance provided by the institution to a country or entity to create new capacity for fossil fuel activity during that year. Further, the bill prohibits the United States from providing any loan, insurance, guarantee, or extension of financial or technical assistance to a country or entity for any fossil fuel activity.