The party won the 1976 general election and formed the first constitutional government after the 1974 revolution, with Mário Soares as prime minister.

The PS lost the 1979 election, but returned to power in 1983, forming, with the Social Democratic Party, a Central Bloc coalition.

The party made a comeback and won a historic absolute majority in the 2005 general election under the leadership of José Sócrates.

Despite losing the 2015 election, the party formed an agreement with the Left Bloc and the Unitary Democratic Coalition and managed to appoint António Costa as Prime Minister.

The Portuguese Socialist Action (ASP) was founded in November 1964, in Geneva, Switzerland, by Mário Soares, Manuel Tito de Morais and Francisco Ramos da Costa.

The ASP was founded in exile by several Socialist members as political organizations during Salazar's Estado Novo regime were forbidden.

[8] Inspired by May 68 events,[9] the Socialist Party (PS) was created at a conference of the Portuguese Socialist Action (ASP) on 19 April 1973, in Bad Münstereifel in West Germany: The twenty-seven delegates decided to found a party of socialism and political freedom, making an explicit reference to a classless society and with Marxism as a source of principal inspiration.

However, seven delegates voted against the idea of creating a party, including Mário Soares' wife Maria Barroso.

On 25 April 1974, the Carnation Revolution brought down the authoritarian regime of the Estado Novo, established in 1933, and democracy was restored.

They won the 1983 general election but without an absolute majority, and the PS formed a grand coalition with the centre-right Social Democratic Party (PSD), creating a Central Bloc.

In 1985, the Central Block broke down and the PS, at the time led by António de Almeida Santos, lost the 1985 legislative election.

In the elections, the PS suffered a huge setback, with 28.1 percent of the vote, ten points behind the PSD, who formed another coalition government with the CDS–PP.

[20] Costa led a very successful first term as prime minister with a growing economy, low unemployment, and deficit cuts.

For the 2021 Portuguese presidential election, Costa endorsed the incumbent Marcelo Rebelo de Sousa, something that made some party members unsatisfied.

[24][25] With the support of the left faction of the party and some more moderate members worried about corruption, Gomes finished in a disappointing second place behind de Sousa, who had many endorsements of party leaders like Lisbon's Mayor Fernando Medina, Eduardo Ferro Rodrigues, and Carlos César.

[32] Three months later, in the 2024 European Parliament elections, the Socialist Party narrowly defeated the AD coalition, by a 32 to 31 percent margin.

The most notable figures of this factions include the marxist Manuel Serra, who opposed Mário Soares leadership from the left and won 44% of the votes against him,[33] leaving the party and creating the People's Socialist Front afterwards;[34] as well as the MPs Carmelinda Pereira and António Aires Rodrigues, who were the most notable examples of a "trotskyist infiltration on the party", it is noted that this faction represented 25% of the delegates elected to the socialist national congress of 1976.