The People's Unity Party – Socialist Party (Icelandic: Sameiningarflokkur alþýðu – Sósíalistaflokkurinn), generally referred to as the Socialist Party (Icelandic: Sósíalistaflokkurinn), was an Icelandic political party that functioned from 1938 to 1968, when the People's Alliance party was created.

It was formed through the merger of the Communist Party of Iceland (KFI) and another splinter group from the Social Democratic Party on the grounds that a united front was needed against fascism, according to the guidelines put forth by the Comintern. The formation of the Socialist Party was a result of the Popular Front line embraced by ComIntern, and promoted in Iceland by Einar Olgeirsson of the KFI. Philosopher Jon Olafsson has argued that as it resulted in a split within the larger Social Democratic Party, Comintern did not approve of the merger in its eventual form.^[1] This has been contested by historians Thor Whitehead and Hannes H. Gissurarson in books about the communist movement.^[2]

The Socialist Party maintained cordial relations with the Communist Party of the Soviet Union. In 1939 the chairmanship was taken over by Einar Olgeirsson, after the previous non-communist chairman had left the party in protest against the refusal of his new allies to condemn the attack by the USSR on Finland in the Winter War.

During the Second World War the influence of the party grew. In the 1942 elections it won over its mother organization, the Social Democrats. The Socialist Party clearly supported the formation of the Icelandic Republic in 1944. During the two first years of independence, the Socialist Party took part in a broad coalition government. The coalition broke down on the issue of the claims of the United States to have a military base in Keflavík, which the socialists opposed.

In 1956, the Socialist Party took part in the People's Alliance along with the Egalitarian Society and Hannibal Valdimarsson. When the People's Alliance was made into a formal political party in 1968, the Socialist Party was merged into it and thus became defunct as an independent entity.