The Revolutionary Socialist League (RSL) was a Trotskyist group^[1] in Britain which existed from 1956 until 1964 when it became Militant, an entryist group in the Labour Party.

After the dissolution of the Revolutionary Communist Party, Ted Grant and his supporters were expelled from the RCP's successor The Club in 1950 and formed the International Socialist Group. They went on to fuse with supporters of the International Secretariat of the Fourth International in Britain as the Revolutionary Socialist League in 1956 and were recognised as the official British section at its fifth world congress in 1957. The RSL held its first congress in 1957.^[2]

It was an entryist group within the Labour Party that published Socialist Fight. In 1958, the group was recognised as the British section of the International Secretariat of the Fourth International and, after the reunification in 1963, the British section of the Fourth International. However, the League registered substantial political differences at the 1965 World Congress, and failed to integrate other supporters of the International in Britain. The Congress recognised two sympathising sections in Britain: the RSL and what became the International Marxist Group, prompting the RSL to turn its back on the International. In 1964, the RSL founded the newspaper Militant and the group itself soon became known by this name, although the official name was still used internally.