Republic of Austria v. Altmann, 541 U.S. 677 (2004), was a case in which the Supreme Court of the United States held that the Foreign Sovereign Immunities Act, or FSIA, applies retroactively to acts prior to its enactment in 1976.

Maria Altmann was an Austrian-American Jewish refugee from Austria, who fled her home country after it was annexed to the Third Reich. She is noted for her ultimately successful legal campaign to reclaim from the Government of Austria five family-owned paintings by the artist Gustav Klimt that were stolen by the Nazis during World War II.