This article studies the concept of solidarity and the definition of international solidarity in congress materials dated 1972-1985 from the Swedish Social Democratic Workers' Party (Sveriges Socialdemokratiska Arbetareparti, SAP) and its three branch organizations. On the one hand, it examines the different definitions and functions of the concept within the party at large, thereby adding to historical research on the recent history of Swedish social democracy. In source material published by youth, women's, and religious wings affiliated with the SAP, the struggle for the meaning of the concept should become apparent, because these organizations represent both the traditional and official concept of solidarity that had been dominant in social democratic rhetoric, as well as the new, challenging concept of solidarity associated with the emerging solidarity movements of the 1970s. On the other hand, the article has a conceptual historical and theoretical aim, intending to understand how solidarity functions as a political and ideological concept for Swedish social democrats during the studied period of time. The study shows that solidarity as an ideological concept had a unifying function for Swedish social democracy. In the concept of solidarity and in the definition of international solidarity, the relevance and topicality of social democratic politics could be emphasized. This was done temporally by giving the contemporary political relevance of solidarity a historical dimension by presenting solidarity in general and international solidarity in particular as a social democratic concept with roots in the early years of the movement. By accentuating the history of the concept, solidarity, despite its obvious contemporary political link, was presented as a timeless social democratic concept, as central in the early twentieth century as in the 1970s and in the future. Thus, in a conceptual struggle with contemporary movements for solidarity and in polemic with the bourgeoisie's attempts to relegate social democracy to the past, the relevance and topicality of social democratic politics was asserted through the concepts of solidarity and, in particular, the definition of international solidarity.