The Popular Socialist Party () was a Spanish political party which emerged in 1968 as 'Partido Socialista del Interior' or the Socialist Party of the Interior. During the Francisco Franco dictatorship it was an illegal underground movement on University Campuses, and it adopted the Popular Socialist Party name in 1974. Its President was the University Professor, Enrique Tierno Galván. That same year the party formed the Spanish Democratic Junta (Junta Democrática de España) together with the Communist Party of Spain (PCE) and Carlist Party.
It defined itself as Marxist and stood in the Spanish general election, 1977 in coalition with a number of regional left wing movements especially the Socialist Party of Andalusia (where they stood under the name 'Socialist Unity'). Overall the PSP won 816,582 votes (4.46%) and 6 deputies. In February 1978 however they decided to merge with the Spanish Socialist Workers' Party (PSOE) and the following year Tierno Galván was elected Mayor of Madrid by the PSOE and PCE.