leftist party in Japan that supports an evolving socialized economy and a neutralist foreign policy.

Japan's first socialist parties appeared in the mid-1920s; moderate factions of the country's labour movement combined to form the Social Mass Party (Shakai Taishūtō) in 1932. The left failed to elect many candidates before World War II, and all of Japan's traditional parties were dissolved in 1940.

In the fall 1945, shortly after the war ended, Japanese political parties began to re-form under the Allied occupation. The Japan Socialist Party (JSP) was set up in November by adherents of three or four prewar proletarian parties. In 1947 the party won 26 percent of the vote and formed a coalition government with the centrist Democratic Party (Minshutō).

This period in power broke the coalition and weakened the JSP; in 1951 they split into left and right socialist parties, and each won roughly 13 percent of the vote until the two wings rejoined in 1955. The union lasted until 1959, when the party again split into the left-wing JSP and the right-wing Democratic Socialist Party (Minshu Shakaitō).

From the 1960s, the JSP—since 1991 called the Social Democratic Party of Japan—though clearly a minority party, dominated Japanese reform politics, generally winning about 20 to 30 percent of the seats in the House of Representatives. From 1986 to 1991 Doi Takako served as chairman of the party, the first woman to head a major political party in Japan. From the mid-1980s the party's support was less consistent, though it was a member of several coalitions that supplanted the Liberal-Democratic Party (Liberal-Democratic Party of Japan)'s (LDP's) monopoly on power in the 1990s. In 1994–96 party chairman Murayama Tomiichi was the first socialist prime minister of Japan since 1948. In 1996, however, the party was reduced to 15 seats in the House of Representatives, though it lent the governing LDP support from outside the government. Its representation was reduced even further in subsequent elections; in 2003, for example, the party won only 5 percent of the vote and 6 seats in the House of Representatives.