Mitsui Koji ( March 6, 1910 - April 20, 1979 ) was a Japanese actor. His real name was Mitsui Hikohide. After the war, he played an active role as a supporting actor in films directed by Akira Kurosawa and Yasujiro Ozu . His former stage name was Mitsui Hideo.

Changed his stage name to Mitsui Koji in Ozu's Hen in the Wind (1948) . His role as the playboy Kisaburo in Kurosawa Akira 's The Lower Depths (1957) won him the Best Supporting Actor award at the 12th Mainichi Film Awards and the 8th Blue Ribbon Awards, solidifying his status as a supporting actor.

After going freelance in 1960, he worked not only in films but also in television dramas and on stage, but after undergoing surgery for a stomach ulcer in 1971, his health deteriorated and he scaled back his acting career.

He died of heart failure on April 20, 1979 at a hospital in Kamakura. He was 69 years old.