The Left Socialist Party of Japan (社会党左派, Shakaitō-saha) was a political party in Japan that existed between 1951 and 1955.^[1]

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Following the signing of the San Francisco Peace Treaty in 1951, the Japan Socialist Party dissolved into chaos and internal bickering between moderate reformist socialists and more radical revolutionary socialists over the issue of whether or not to support the Treaty. The JSP split, with some of its members forming a more centrist social-democratic party, while others formed a more radical socialist party. Both groups claimed the name Nihon Shakaitō (日本社会党) but different English translations, and are known as the Left Socialist Party of Japan and the Right Socialist Party of Japan, respectively.

The left-wing in Japan was in chaos between 1951 and 1955. In early 1955, the Left and Right 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 Left Socialists generally had the upper hand in the reunified JSP, causing a few former Right Socialists to leave the party in 1960 to create the Democratic Socialist Party.

• Itsurō Sakisaka
• List of political parties in Japan
• Politics of Japan