The Socialist Party of Missouri was the Missouri state section of the Socialist Party of America (SPA), an organization originally established in 1901 as a federation of semi-autonomous state organizations. The Missouri organization was the direct lineal descendant of the Social Democratic Party of Missouri, which emerged in 1898.
The Socialist Party of Missouri was centered in the city of St. Louis, which was at the time of formation of the SPA the fourth most populous city in America and home to a large German-American immigrant community. Growth of the party was bolstered by the newspaper St. Louis Labor, a prominent weekly with a significant circulation among the organized labor movement of the city.
The Socialist Party of Missouri remained active throughout the Great Depression decade of the 1930s and into the 1940s, before atrophying and losing critical mass. The party's archives were donated to the Missouri Historical Society in 1964.