Japan Socialist Party

The Japan Socialist Party (日本社会党, Nihon Shakai-tō, abbr. JSP) was a socialist and progressive political party in Japan that existed from 1945 to 1996. The party was founded as the Social Democratic Party of Japan by members of several proletarian parties that existed before World War II, including the Social Mass Party, the Labour-Farmer Party, and the Japan Labour-Farmer Party. The party represented the Japanese left after the war, and was a major opponent of the right-wing Liberal Democratic Party.

The JSP was briefly in power from 1947 to 1948. From 1951 to 1955, the JSP was divided into the Left Socialist Party and the Right Socialist Party. In 1955, Japan's two major conservative parties merged to form the Liberal Democratic Party (LDP), establishing the so-called 1955 System, which allowed the party to near-continuously hold power since. The JSP was the largest opposition party but was incapable of forming government. Nonetheless, the JSP managed to hold about one third of the seats in the National Diet during this period, preventing the LDP from revising the Constitution of Japan.

In the late 1980s and early 1990s, the JSP under the leadership of Takako Doi earned a record-high number of seats. However, the establishment and electoral success of new conservative parties in the mid-1990s took the JSP by surprise and its share of seats in the National Diet decreased significantly. The party was formally dissolved in 1996. Its successor is the Social Democratic Party, a minor party holding two parliamentary seats, one each in the Houses of Representatives and Councillors as of 2022.

Two Japanese prime ministers, Tetsu Katayama and Tomiichi Murayama, were members of the JSP.