The Socialist League was a socialist political party in the United Kingdom. It was founded in 1885 by former members of the Social Democratic Federation unhappy with the leadership of H. M. Hyndman.
The initial manifesto of the organisation was composed by William Morris and Ernest Belfort Bax. It advocated revolutionary internationalism, but was not purely Marxist, encompassing Morris' emerging guild socialism. The party published a journal, Commonweal.
Notable members of the party included Eleanor Marx and Edward Carpenter. The party also attracted many anarchists, who gained control by the early 1890s. This led some of the founders to leave and rejoin the SDF, or the Independent Labour Party. Nonetheless, the party remained active until it was disbanded in 1901.
A second, entirely unconnected Socialist League was formed in the 1930s as a split from the Independent Labour Party, opposed to that organisation disaffiliating from the Labour Party. It was led by Stafford Cripps.