Seiyo is a city located in Ehime Prefecture, Japan. It is situated in the southwestern part of the prefecture, on the island of Shikoku.
As of 31 August 2022, the city had an estimated population of 35,456 people living in 17,627 households, giving it a population density of 69 persons per square kilometre. The total area of the city is 514.34 square kilometres (198.59 square miles).