Sen Katayama (?? ? Katayama Sen, December 26, 1859 – November 5, 1933), born Yabuki Sugataro (?? ??? Yabuki Sugataro), was an early member of the American Communist Party and co-founder, in 1922, of the Japanese Communist Party.
After 1884, he spent most of his life abroad, especially in the United States and the Soviet Union, where he was very active in the international socialist community, and after 1920 the communist community.
Katayama had a weak base inside Japan, and was little known there.
However, in the rest of the world, he was widely hailed as a leading spokesman for the Japanese socialist and communist movements.