The Fourth International Posadist is a Trotskyist international. It was founded in 1962 by J. Posadas, who had been the leader of the Latin America Bureau of the Fourth International in the 1950s, and of the Fourth International's section in Argentina. Between their split from the International Secretariat of the Fourth International in 1962 and Posadas' death in 1981, Posadists developed a strain of communism that included several fringe ideas, which brought them into conflict with more mainstream left-wing groups.

Posadism attempts to introduce elements of Ufology into Marxist thought. Arguing that only communism can allow the development of interplanetary travel, they concluded that visiting aliens from other planets must live in highly advanced communist societies and are bound to help Earth-based communists with bringing about the world revolution.