The Socialist League was an early revolutionary socialist organisation in the United Kingdom. The organisation began as a dissident offshoot of the Social Democratic Federation of Henry Hyndman at the end of 1884. Never an ideologically harmonious group, by the 1890s the group had turned from socialism to anarchism, and disbanded in 1901.

The Social Democratic Federation (SDF) was established as Britain's first organised socialist political party by H. M. Hyndman, and had its first meeting on 7 June 1881. Those joining the SDF included William Morris, George Lansbury, James Connolly and Eleanor Marx. However, Friedrich Engels, Karl Marx's long-term collaborator, refused to support Hyndman's venture. Many of its early leading members had previously been active in the Manhood Suffrage League.