San'yō-Onoda is a city located in Yamaguchi Prefecture, Japan. As of 31 May 2023, the city had an estimated population of 59,867 in 29122 households and a population density of 530 persons per km2. The total area of the city is 133.09 square kilometres (51.39 sq mi).

Yamaguchi Prefecture is a prefecture of Japan located in the Chūgoku region of Honshu. Yamaguchi Prefecture has a population of 1,377,631 and has a geographic area of 6,112 km2. Yamaguchi Prefecture borders Shimane Prefecture to the north and Hiroshima Prefecture to the northeast.