Suehiro Nishio was a Japanese labor activist and party politician whose career extended across the prewar and postwar periods. A long-serving member of the National Diet, he was a power broker in the Japan Socialist Party and one of the main leaders of the Right Socialists. He served as Deputy Prime Minister of Japan during the cabinet of Hitoshi Ashida, and in January 1960, he led a breakaway faction out of the Japan Socialist Party to found the new Democratic Socialist Party.

The Japan Socialist Party was a socialist and progressive political party in Japan that existed from 1945 to 1996. The party was founded as the Social Democratic Party of Japan by members of several proletarian parties that existed before World War II, including the Social Mass Party, the Labour-Farmer Party, and the Japan Labour-Farmer Party. The party represented the Japanese left after the war, and was a major opponent of the right-wing Liberal Democratic Party.