The International Socialist League is a small Trotskyist organisation in Britain. It has a regular publication called Socialist Voice.

The group's origins lie in the disintegration of the Workers Revolutionary Party (WRP) in the 1980s. It was founded as the Bolshevik Faction of Cliff Slaughter's WRP in August 1997 and from the start was sympathetic to the Latin American-based Trotskyist leader Nahuel Moreno.

In February 1998 the future ISL split from the WRP and under the leadership of the veteran Bill Hunter and Martin Ralph founded their organisation, which affiliated to Moreno's International Workers League (LIT).

The ISL remains active in the North West of England where its small membership is concentrated and it was active in the Socialist Alliance. It is supportive of the Socialist Alliance Democracy Platform and the Merseyside-based Campaign for a new Mass Workers Party.