The Japan Socialist Party (日本社会党, Nihon Shakai-tō; JSP) was a major socialist and progressive political party in Japan which existed from 1945 to 1996. The party was the primary representative of the Japanese left and main opponent of the right-wing Liberal Democratic Party for most of its existence.
The JSP was founded in 1945 by members of pre-war proletarian parties, including the Shakai Taishūtō. In the 1947 election, the JSP became the largest party in the National Diet and formed a government under Tetsu Katayama until 1948. From 1951 to 1955, the JSP was split into the Left Socialist Party and the Right Socialist Party, and in 1960 some of its members broke away to form the rival Democratic Socialist Party. In 1955, Japan's two major conservative parties merged to form the Liberal Democratic Party (LDP), which has held power near-continuously since. The JSP was the largest opposition party for the next 40 years, but was incapable of forming a government. Nonetheless, it 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.