The Socialist International is a worldwide organization of social democratic, labor, and democratic socialist political parties. It draws its name from the original Socialist International (often called the Second International), which was formed in 1889. Among the Second International's most famous actions were its (1889) declaration of 1st May as International Labour Day and its (1910) declaration of 8th March as International Women's Day. The Second International was split by the outbreak of World War I, then re-formed in 1923 (as the Labour and Socialist International), and finally reconstituted again, in its present form, after World War II (during which many social democratic and socialist parties had been suppressed in Nazi-occupied Europe).

During the post-World War II period, the SI aided social democratic parties in re-establishing themselves when dictatorship gave way to democracy in Portugal (1974) and Spain (1975). Until its 1976 Geneva Congress, the Socialist International had few members outside Europe and no formal involvement with Latin America<ref>The Dictionary of Contemporary Politics of South America, Routledge, 1989</ref>. In the 1980s, most SI parties gave their backing to the Nicaraguan Sandinistas (FSLN), whose left-wing government had incited enmity from the United States. Since then, the SI has admitted as member-parties not only the FSLN but also ex-Communist parties such as the Italian Democrats of the Left (Democratici di Sinistra (DS)) and the Front for the Liberation of Mozambique (FRELIMO).

The Party of European Socialists, a European political party active in the European Parliament, is an associated organization of the Socialist International.

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Presidents

As of 2006, George Papandreou, leader of the Panhellenic Socialist Movement, is the president of the Socialist International.