San'yō-Onoda (山陽小野田市, San'yō-Onoda-shi) is a city located in Yamaguchi Prefecture, Japan.

It is long in the north-south direction and has a fan shape that opens to the Seto Inland Sea.

Following the Meiji restoration, the villages of Asai, Sue, and Takachiho were established within Asa District, Yamaguchi with the creation of the modern municipalities system on April 1, 1889.

San'yō-Onoda has a mayor-council form of government with a directly elected mayor and a unicameral city council of 22 members.

In terms of national politics, the city is part of the Yamaguchi 3rd district of the lower house of the Diet of Japan.