Socialist Party, political party of the United States, founded in Indianapolis, Indiana, in 1901. The first political party in the U.S. dedicated to the promotion of socialism was the Socialist Labor party, founded in 1877. In 1890 leadership of this party was assumed by Daniel De Leon, an authoritarian follower of Karl Marx's revolutionary policies.

In 1899, moderate members of the Socialist Labor party, led by the American lawyer Morris Hillquit, resigned. Meanwhile, in 1898, the Social Democratic party had been founded by the American labor leader Eugene V. Debs and the American editor and legislator Victor Berger. This party had some early success in local elections in Massachusetts, and Debs received about 96,000 votes as its presidential candidate in 1900. The American Congregationalist minister George Davis Herron became a socialist in 1899, hoping to give the movement a Christian orientation. In 1901 Hillquit and his faction of the Socialist Labor party joined Debs, Berger, and other Social Democrats and the Christian Socialists to form the Socialist Party of America. By 1912 party membership had increased to approximately 118,000. Debs, presidential candidate of the party in 1904 and 1908, received 900,672 votes (6 percent of the popular vote), in the 1912 presidential election. In that year the party had more than 1000 members in public office. The reformist policies, or “immediate demands,” of the party, dedicated to achieving socialism through peaceful democratic methods, were disseminated by influential publications. The party also played an important role in the growth of trade unions in the U.S.

The Socialist party denounced World War I and the belligerent role of the United States in what it regarded as an imperialist conflict, although some of the party's leaders resigned to support the war. This antiwar stance was one factor in the party's undoing. Debs was arrested in Canton, Ohio, for criticizing the war effort and sentenced to ten years in prison under the Espionage Act of 1917. Dozens of like-minded Socialists were jailed under the Sedition Act of 1918. In 1920, while in prison, Debs was again the party candidate for the presidency. He received 919,799 votes, the largest vote ever cast for a presidential candidate of the Socialist party. Meanwhile, the Russian Revolution of 1917 led to a split in the party. The left wing, which later came to constitute the Communist party, advocated similar revolutionary methods and recommended the establishment of a workers' dictatorship in the U.S. Following the party split, in 1919, the Socialist party declined in membership to approximately one-fourth its former size.