The United States Supreme Court recently employed foreign legal sources to interpret U.S. law, provoking widespread political and legal controversy. Scholars have yet to examine systematically the conditions under which justices cite foreign law, however. Applying theoretical approaches from international relations and judicial politics scholarship, we search every Supreme Court opinion between 1953 and 2009 for references to foreign law. Justices strategically reference foreign law to prop up their most controversial opinions. They also borrow law from countries whose domestic political institutions are viewed as legitimate, and, surprisingly, conservatives are as likely as liberals to cite foreign law. These findings add important information to the discussion over citing foreign law, and highlight how geopolitical context influences domestic legal policy.