The Rightist Socialist Party of Japan (, the official English name was Social Democratic Party of Japan) was a Japanese political party that existed between 1948 and 1955. It was a centre-left political party, which adopted a policy of moderate social-democracy.
Following the defeat of the Social Democratic Party of Japan (SDPJ) in 1948 at the hands of Japan's two main conservative parties, the Liberal Party and the Democrat Party, the SDPJ dissolved into chaos and internal bickering between moderates and Marxist-Leninists. As a result of the SDPJ split, some of its members formed a moderate, almost centrist social-democratic party, while others formed a more radical, socialist, Marxist-Leninist party. Both groups claimed the name "" but different English translations 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 the merger of the Liberal and Democrat Parties. Even though the Rightist Socialist Party dissolved in 1955 when the JSP reunified, some members of the former Rightist Socialist Party broke off from the JSP in 1960 and created the Democratic Socialist Party. A newly formed youth organisation, Young Socialists (which retains full membership in the International Union of Socialist Youth), is said to be inherited from the political tradition of the Rightist Socialist Party.