Masashi Ishibashi was a Japanese politician who served as chairman of the Japan Socialist Party from 1983 to 1986.

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.