Juan Posadas (died 1981) (real name Homero Cristali) was an Argentinian Trotskyist. His group was called the Quatrième Internationale Posadiste.
As well as being a Trotskyist leader he is known for his theories on UFOs. He believed that the existence of UFOs demonstrated the existence of socialism on other planets. He argued that only a socialist society could produce the technology needed for interplanetary travel.
Posadas became a Trotskyist in Argentina before World War Two. He became prominent when the Fourth International split into two competing groups and Posadas became the leader of the Argentinean section of the International Secretariat of the Fourth International.
