The Tsuyama massacre was a revenge spree killing that occurred on the night of 21 May 1938 in the rural village of Kamo close to Tsuyama in Okayama, Empire of Japan. Mutsuo Toi , a 21-year-old man, killed 30 people, including his grandmother, with a Browning shotgun, katana, and axe, and seriously injured three others before killing himself with the shotgun. It is the deadliest shooting by a lone gunman in Japanese history.

Okayama Prefecture is a prefecture of Japan located in the Chūgoku region of Honshu. Okayama Prefecture has a population of 1,906,464 and has a geographic area of 7,114 km2. Okayama Prefecture borders Tottori Prefecture to the north, Hyōgo Prefecture to the east, and Hiroshima Prefecture to the west.