Seiyo (西予市, Seiyo-shi) is a city located in Ehime Prefecture, Japan.

Seiyo is located in southeastern Ehime Prefecture, with Uwa Bay on the Seto Inland Sea to the west, and with the Shikoku Mountains to the east, with a difference in elevation of 1403 meters.

Ehime Prefecture Kōchi Prefecture Seiyo has a diverse climate, from the inland mountain plateau (Onogahara), where more than 2 meters of snow accumulates in winter, to the warm coastal area, and the basin with a lot of fog due to the influence of two dams.

In general, Seiyo has a humid subtropical climate (Köppen Cfa) characterized by warm summers and cool winters with light snowfall.

Seiyo has a mayor-council form of government with a directly elected mayor and a unicameral city council of 18 members.

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

It never achieved any measure of popularity, and the owner eventually donated it to the city of Seiyo.