Yoichi Sai (born 6 July 1949 in Nagano Prefecture, Japan) is a Japanese film director. His mother is Japanese and his father is Zainichi Korean（Korean Japanese）.
Sai's 2004 film Chi to hone won four Japanese Academy Awards, including two for Sai himself, for Best Director and Best Screenplay. He had previously received two nominations in the same categories for Tsuki wa dotchi ni dete iru. In 1999 he shot Buta no mukui (The Pig's Retribution), a film set in the lavish natural scenery of Okinawa, inspired by the 1996 Akutagawa Prize-winning eponymous novel by Eiki Matayoshi. The film won the Don Quixote prize at Locarno International Film Festival in 1999.
Sai won the award for Best Screenplay at the 11th Yokohama Film Festival for A Sign Days.
As an actor, he appeared in Nagisa Oshima's 1999 film Taboo. He is the current president of the Directors Guild of Japan.