American socialist, one of the founders of the Industrial Workers of the World (IWW). He was one of the chief propagandists for socialism in the early American labour movement, but his uncompromising tactics were often divisive.

De Leon arrived in the United States in 1874. In 1890 he joined the Socialist Labor Party. Within a few years he became one of the leading figures in the party, editing its newspaper and helping to transform it into a disciplined national organization. He excoriated the labour union leadership of the day as insufficiently radical and in 1895 led a faction that seceded from the Knights of Labor, subsequently forming the Socialist Trade and Labor Alliance (STLA). In 1899 a dissident faction left the SLP and formed what became the Socialist Party of America. The membership and prestige of the SLP declined thereafter.

At a convention in Chicago in 1905, De Leon helped found the IWW, with which the STLA promptly merged. But he was refused a seat at the IWW's 1908 convention by extremists who rejected political activity of the sort that he advocated and who favoured more violent tactics. He then created another schismatic body, the Workers' International Industrial Union, which failed.