PSOE was founded with the purpose of representing and defending the interests of the working class formed during the Industrial Revolution in the 19th century. In its beginnings, 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 socialising the means of production in order to establish a dictatorship of the proletariat in the transition to socialist society. The ideology of the Spanish Socialist Worker's Party has evolved throughout the 20th century according to relevant historical events and the evolution of Spanish society.
In 1979, the party abandoned its definitive Marxist theses at the hands of its then secretary general Felipe González, not before overcoming great tensions and two congresses, the first of which preferred to maintain Marxism. Before this situation, notable internal leaders like Pablo Castellano or Luis Gómez Llorente founded the internal faction of Left Socialists, which included the militants who would not renounce Marxism. This allowed for the consolidation of the leftist forces in PSOE. From this moment, the diverse events both outside and within the party led to projects that resembled those of other European social democratic parties and acceptance of the defence of the market economy.
Currently, PSOE defines itself as "social democratic, centre-left and progressive". It is grouped with other self-styled socialists, social democrats and labour parties in the Party of European Socialists.
During the Second Republic the matter of the conception of the State was open within the party: two different views connected in discourse to the interests of the working class competed against each other, a centralist view as well as a federal one. The late years of the Francoist dictatorship was a period in which PSOE defended the right to "self-determination of the peoples of Spain", in what it was a reflection of both an ideologic and a pragmatist approach. Ultimately, the party, while sticking to a preference for a federal system, gradually ceased to mention the notion of self-determination during the Spanish transition to democracy. Postulates coming from peripheral nationalisms that have been assumed by elements of the party, bringing an understanding of Catalonia, the Basque Country and Galicia as nations and thus, deserving of a different treatment than the rest of regions, have been heavily criticised by other party elements, as according to the later, they would undermine the principle of territorial equality among the autonomous communities.