Sen Katayama (片山 潜, Katayama Sen, December 26, 1859 – November 5, 1933), born Yabuki Sugataro (藪木 菅太郎, Yabuki Sugatarō), was an early Japanese Marxist political activist and journalist, one of the original members of the American Communist Party and co-founder, in 1922, of the Japanese Communist Party.

In his autobiography, Jiden (自伝), Katayama admitted that he was fortunate not to have been the first born in his birth family, as it saved him from some of the responsibilities that burdened some of his acquaintances.

In 1878, Katayama travelled to Tokyo to apprentice as a printer while studying at a small preparatory school, the Oka-Juku, where he formed a friendship with Iwasaki Kiyoshichi (岩崎清七), nephew of one of the founders of Mitsubishi.

When his crop failed he became employed by a Japanese restaurant owner in Houston, Tsunekichi Okasaki, who bought 10,202 acres (41.29 km2) of land in Texas with the plan that Katayama farm it.

In late 1905, the two borrowed $100,000 from Iwasaki to fund the rice harvest, together forming a "Nippon Kono Kabushiki-gaisha" (Japan Farming Company) to develop the project, with Katayama as managing director.