political leader who played a significant role in the development of Spanish democratic socialism and trade unionism.

Iglesias was raised in a foundling home and eventually became a printer. He helped found the Spanish Socialist Workers' Party (Partido Socialista Obrero Español; PSOE) in May 1879 and soon became its secretary. In 1882 he organized the first strike in Spain after the 1875 restoration of the monarchy, and in 1885 he became president of the PSOE's central committee. The following year El Socialistica, the socialist newspaper, was founded, with Iglesias as editor. He also headed the socialist-affiliated Unión General de Trabajadores (General Union of Workers), organized in 1888.

An effective organizer, Iglesias guided the slowly expanding PSOE on a disciplined, austere, and evolutionary course. Although for many years he scorned alliance with non–working-class parties, he believed in parliamentary and municipal political action. He was one of the first socialists elected to the Madrid Municipal Council (1905) and to the Cortes, the Spanish parliament (1910). In 1921 he helped prevent the PSOE from joining the Third International.