Lieto (Swedish: Lundo) is a municipality in the region of Lounais-Suomi. It is located in the province of Western Finland and is part of the Southwest Finland region. The municipality has a population of 19,846 (31 January 2019) and covers an area of 302.56 square kilometres (116.82 sq mi) of which 1.66 km2 (0.64 sq mi) is water. The population density is 99.97 inhabitants per square kilometre (258.9/sq mi).
The municipality is unilingually Finnish.
Lieto has a medieval stone church, St. Peter's church, that originates from around 1500 near the city center.