The Socialist Party of Great Britain (SPGB) is a socialist political party in the United Kingdom. Founded in 1904 as a split from the Social Democratic Federation (SDF), it advocates using the ballot box for revolutionary purposes and opposes both Leninism and reformism. It holds that countries which claimed to have established socialism had only established "state capitalism" and was one of the first to describe the Soviet Union as state capitalist. The party's political position has been described as a form of impossibilism.

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.