The National Socialist Party was a small political party in Britain, founded in 1916. It should not be confused with the German NSDAP, which was created long after the British NSP. At the time when the NSP was established, the term "national socialism" carried none of its present-day right-wing connotations. The British NSP had broadly leftist policies and was in no way associated with the doctrine of Nazism (in fact, the NSP was dissolved by the time German Nazism began to emerge).

The National Socialist Party party was founded by H. M. Hyndman and his followers after his defeat in the leadership elections of the British Socialist Party. They believed that it was desirable to support Britain in World War I against "Prussian militarism". Although maintaining that they were a Marxist party, after affiliation to the Labour Party in 1918, they renounced vanguardism and saw in the Russian Revolution only the danger that it might weaken Britain's war effort.

Six members of the party were elected to parliament in the 1918 election. In 1919, it changed its name to the Social Democratic Federation, and at one point had eleven MPs, but after the death of Hyndman in 1921, the group gradually dissolved into the Labour Party.