The British Socialist Party was a socialist party founded in Britain in 1911. The founding conference, called by the Social Democratic Party (better known by their earlier name, the Social Democratic Federation) also drew some Independent Labour Party branches and groups adhering to the Clarion newspaper, alongside individuals and representatives of smaller socialist groups.

The former SDF, led by H. M. Hyndman, rapidly took control of the new organisation, and advocated political action over and above industrial or trade union action. Nonetheless, John Maclean, the party's leader in Scotland, played a leading role in Red Clydeside strikes during World War I.

Hyndman had always taken a nationalist viewpoint, and advocated funding the British military to oppose Prussian belligerency. This proved increasingly controversial within the BSP, and led to a number of small revolts gaining greater rank-and-file control over party procedures.

In 1914, the party, keen to join the Second International despite its obvious reformism, voted to affiliate to the Labour Party. But by this time, the party was on the verge of splitting over attitudes to the war.

The most right-wing section of the party split in early 1915 to form the Socialist National Defence League, while the leadership was defeated in elections in 1916 by an internationalist group, essentially pacifist, supporting the programme of the Zimmerwald Conference. Hyndman and his followers established the National Socialist Party.

The party's new leadership maintained the desire to join the Second International, and the BSP was finally accepted into the Labour Party later that year.

By late 1918, many in the party, including Maclean, were inspired by the lead of the Bolsheviks in the Russian Revolution and determined to form a British Communist Party. They negotiated with the Socialist Labour Party, but could not agree on the question of whether the new party should affiliate to the Labour Party, and so formed the Communist Party of Great Britain with some of the SLP's members in 1920, dissolving the party.