Following the defeat of the Japan Socialist Party (JSP) in 1948 at the hands of Japan's two main conservative parties, the Liberal Party and the Democrat Party, the Japan Socialist Party dissolved into chaos and internal bickering between moderates and Marxist-Leninists. As a result the JSP split: some of its members formed a moderate and almost centrist social-democratic party, while others formed a more radical, socialist, and Marxist-Leninist party. Both groups claimed the name "Japan Socialist Party" and are known as the Rightist Socialist Party of Japan and Leftist Socialist Party of Japan, respectively.
The left-wing was in chaos between 1948 and 1955, and in early 1955 the Rightist Socialists and the Leftist Socialists reconciled and merged to reform the JSP, months before the Liberal Democrat Party was created through a merger of the Liberal and Democrat Parties. The Leftist Socialists generally had the upper hand in the reunified JSP, causing a few rightists to leave the Party in 1960 to create the Democratic Socialist Party.
On domestic policy, the party was radical socialist, Marxist-Leninist and left-wing. It is now defunct.