The area of San'yō-Onoda was part of an ancient
Nagato Province. During the
Edo Period, the area was part of the holdings of
Chōshū Domain. 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. Asai became the town of Asa on October 1, 1918 and Sue was elevated to town status on April 3, 1920 and renamed
Onoda. Takachiho was elevated to town status on April 1, 1938 and merged with Onoda on November 3, 1940 to become the city of Onoda. Asa merged with the town of Habu to form the town of
San'yō on September 30, 1956. Onoda and San'yo merged to form the city of San'yō-Onoda on March 22, 2005. ==Government==