In an address to a European think tank in Brussels on Friday, Gates questioned NATO's viability in a post-Cold War era, saying its members' penny-pinching and lack of political will could hasten the end of U.S. support.
The North Atlantic Treaty Organization was formed in 1949 as a U.S.-led bulwark against Soviet aggression.
Gates has made no secret of his frustration with NATO bureaucracy and the huge restrictions many European governments placed on their military participation in the Afghanistan war.
Gates is set to retire at the end of the month. His designated successor, Leon Panetta, is expected to take over July 1.
The Associated Press contributed to this story