Socialist Unity was a Trotskyist group in the United States. It was founded by former members of the Socialist Workers Party in 1985, around Les Evans who wanted to maintain links with the United Secretariat of the Fourth International. Immediately after being expelled from the SWP, future Socialist Unity supporters formed part of the Fourth Internationalist Tendency but left that grouping in favour of joining the Solidarity with the International Socialists and Workers Power in 1986. Within Solidarity they constituted themselves as the Fourth International Caucus and retain links with the United Secretariat of the Fourth International in this capacity.

Socialist Unity was also a small socialist electoral coalition in the United Kingdom. It was formed by the International Marxist Group as a response to the Socialist Workers Party standing candidates in elections.

Initially, in 1977, the IMG formed local groups, and then the national Socialist Unity grouping. They suggested an alliance with the SWP, which was rejected. The only other group to join Socialist Unity was the quasi-Maoist Big Flame which absorbed the remnants of the Libertarian Communist Group at this time.

The group stood several candidates in the 1979 UK general election, but all polled very poorly, and the coalition was dropped when the IMG decided to enter the Labour Party.

The Socialist Green Unity Coalition is standing candidates in the 2005 UK general election as Socialist Unity.

For several similarly-named groups in other countries, see Socialist Unity Party.