(dē lē'ŏn), 1852–1914, American socialist leader. Born on the island of Curaçao of Spanish-American parents, he was educated in Germany and the Netherlands before going (1872) to New York City. There he edited a Spanish newspaper, studied law at Columbia (LL.B., 1876), practiced law for a few years, and then returned to Columbia to lecture (1883–89) on Latin American diplomacy. His interest in labor reform grew, and he joined successively the Knights of Labor (1888), Edward Bellamy's “Nationalist” movement (1889), and the Socialist Labor party (1890).
De Leon was the Socialist Labor candidate for governor of New York in 1891, and for years he edited the Socialist Labor weekly, The People. He was an inflexible and doctrinaire Marxian revolutionist and consequently fell out with most other liberal leaders. He opposed unionization of labor according to trades and led the group that formed the Socialist Trade and Labor Alliance, but his leadership was too radical for some of the members (prominent among them Morris Hillquit), who withdrew in 1899 and ultimately formed the Socialist party.
De Leon's prestige subsequently lessened. He helped to found the Industrial Workers of the World in 1905, but in the quarrel over political action he and his followers were expelled. The rival Workers' International Industrial Union, which he then organized, did not flourish. He wrote a great deal of Socialist polemical literature and translated a work of Karl Marx.