Albert Einstein became a United States citizen in 1940.
Einstein was born a German citizen, later became Swiss, and briefly reacquired German citizenship when he moved to Berlin in 1914. After Adolf Hitler came to power in 1933, Einstein renounced his German citizenship and decided not to return to Germany. He settled in the United States and joined the Institute for Advanced Study in Princeton, New Jersey.
Einstein filed a declaration of intention for U.S. citizenship in 1936. The naturalization process took several years, and he became an American citizen in 1940 while retaining his Swiss citizenship. He remained associated with Princeton and the Institute for Advanced Study until his death in 1955.
A common confusion is 1933, the year Einstein left Germany and began his permanent American residence. Another is 1936, the year he filed his declaration of intention. Neither is the year he actually became a U.S. citizen.