The PSOE has been in government longer than any other political party in modern democratic Spain: from 1982 to 1996 under Felipe González, 2004 to 2011 under José Luis Rodríguez Zapatero, and since 2018 under Pedro Sánchez.

They have denounced policies including deregulation and the increase in precarious and temporary work, cuts in unemployment and retirement benefits, and the privatisation of big state-owned organisations and public services.

[12] Same-sex marriage and adoption were legalised in 2005 under the Zapatero Government and, more recently, a transgender rights bill was passed to allow more freedom in regards to gender identity.

The PSOE was founded by Pablo Iglesias on 2 May 1879 in the Casa Labra tavern in Tetuán Street near the Puerta del Sol at the centre of Madrid.

The bulk of the growth of the PSOE and its affiliated trade union, the Unión General de Trabajadores (UGT) was chiefly restricted to the Madrid-Biscay-Asturias triangle up until the 1910s.

[19] The obtaining of a seat at the Congress by Pablo Iglesias at the 1910 Spanish general election in which the PSOE candidates presented within the broad Republican–Socialist Conjunction became a development of great symbolical transcendence and gave the party more publicity at the national level.

[21] The members of the organizing committee (Julián Besteiro, Francisco Largo Caballero, Daniel Anguiano and Andrés Saborit) were accused of sedition and sentenced to life imprisonment.

[26] The last years of the dictatorship saw a divergence emerge among the corporatist which was personified by Francisco Largo Caballero, who began to endorse the rapport with bourgeois republicans; and Julián Besteiro, who continued to show great distrust towards them.

[27] Besteiro's refusal to participate in the Revolutionary Committee led to his resignation as president both of the party and the trade union in February 1931.

[29] After the proclamation of the Second Spanish Republic on 14 April 1931, three PSOE members were included in the cabinet of the provisional government, namely Indalecio Prieto (Finance), Fernando de los Ríos (Education) and Francisco Largo Caballero (Labour).

After the November 1933 general election which marked a win for the centre-right forces in a climate of increasing polarization and growing unemployment, along with a desire to make amends for the mistake of not having sided with the republicans in the election against the united right, Largo Caballero adopted a revolutionary rhetoric, calling for violent revolution and a transitionary dictatorship of the proletariat.

[30][31] Indalecio Prieto had also participated in the increasingly aggressive rhetoric, having already condemned the heavy-hand repression of the December 1933 largely anarchist uprising by the government, that has been cheered on by the CEDA leaders on parliament.

[32] The Socialist Youth of Spain (JSE) also engaged into a shrilling revolutionary rhetoric while Besteiro firmly opposed the insurrectionary drift of the militancy.

[36] The UGT called for a nationwide general strike for 5 October which developed into a full-blown insurrection (the Revolution of 1934) in the mining region of Asturias which was vocally supported by socialists such as Largo Caballero and Prieto.

After the end of the revolt, whose repression was entrusted to generals Francisco Franco and Manuel Goded, most PSOE and UGT leaders were jailed.

[37] A growing rift between Prieto and Largo Caballero (with disparate views of politics, albeit sharing a general pragmatist approach) formed in 1935 while Besteiro's hold on the party diminished significantly.

[42] The PSOE congresses in exile during the post-war period were marked by strong anti-communist positions as a reflection of how the exiles remembered the last events of the Civil War (which featured bitter strifes with the communists) and in line with the stance of other parties of the Socialist International during the Cold War, neglecting any kind of rapprochement with the Communist Party of Spain (PCE).

[43] The relative void left in Spain by the PSOE, with a Toulouse-based direction lacking in dynamism and innovation, was filled by the PCE and other new clandestine organizations such as the Agrupación Socialista Universitaria (ASU), the Popular Liberation Front (FELIPE) or later the Enrique Tierno Galván's Socialist Party of the Interior.

In the 2003 Catalan regional election, the PSOE's Socialists' Party of Catalonia (PSC) increased its vote total, but ended up in second place after Convergence and Union.

After waning popularity throughout their second term, mainly due to their handling of the worsening economic climate in Spain in the aftermath of the 2007–2008 financial crisis, the PSOE was defeated in the 2011 Spanish general election by the conservative People's Party.

This party congress was the first to use a primary election system with three candidates, namely Pedro Sánchez, Eduardo Madina and José Antonio Pérez Tapias.

Due to the large increase of parties such as Podemos (left) and Citizens (centre-right), the PSOE got about 20% of the vote, its worst result since democracy was restored.

In mid-2018, the National Court found that the conservative People's Party profited from the illegal kickbacks-for-contracts scheme of the Gürtel case, confirming the existence of an illegal accounting and financing structure that ran in parallel with the party's official one since 1989 and that sentenced that the PP helped to establish "a genuine and effective system of institutional corruption through the manipulation of central, autonomous and local public procurement".

The PSOE Parliamentary Group in the Congress of Deputies filed a motion of no confidence against the government of Prime Minister Mariano Rajoy, presenting Sánchez as alternative candidate.

For most of his first term as prime minister, Sánchez relied on support from the UP and the NC to get his agenda passed, occasionally being forced into negotiating with the Catalan separatist parties the ERC and the PDeCAT and the PNV on individual issues.

PSOE, UP, En Comú Podem, Grupo Común da Esquerda, PNV, Más País, Compromís, NCa, the Galician Nationalist Bloc (BNG) and Teruel Existe (TE) voted in favor of the government, with PP, Vox, Cs, Together for Catalonia (JxCat), the Popular Unity Candidacy (CUP), NA+, CC, PRC and FAC voting against while ERC and EH Bildu both abstained.

[citation needed] In its beginnings, the PSOE's main objective was the defense of worker's rights and the achievement of the ideals of socialism, emerging from contemporary philosophy and Marxist politics, by securing political power for the working class and establish a dictatorship of the proletariat in order to achieve social ownership of the means of production.

The democratic socialist faction has been especially critical of the party's Third Way move to the centre starting in the 1980s for its economic liberal nature, denouncing the policies of deregulation, cuts in social benefits, and privatisations.

[57] The later years of the Francoist dictatorship saw a period in which the PSOE defended the right to "self-determination of the peoples of Spain", as a reflection of a newer ideological and a pragmatist approach of the party.

[58] Ultimately, the party, while sticking to their preference for a federal system, gradually ceased to mention the notion of self-determination during the Spanish transition to democracy.