Plainville is a town in Hartford County, Connecticut, United States. The population was 17,716 at the 2010 census.
Plainville first was inhabited by Europeans around 1650. By the 1660s the land was incorporated as land for nearby Farmington. In the year 1869, it separated from Farmington due to the distance of the town center and the growth of Plainville downtown due to the installation of the New York, New Haven and Hartford Railroad and the Hartford, Providence, and Fishkill Railroad.
According to the United States Census Bureau, the town has a total area of 25.3 square kilometres (9.8 sq mi), of which 9.7 square miles (25.2 km2) is land and 0.077 square mile (0.2 km2, or 0.72%, is water. The east side of the town is bordered by two prominent peaks of the Metacomet Ridge: Pinnacle Rock and Bradley Mountain. The 51-mile (82 km) Metacomet Trail traverses those peaks.
As of the census of 2000, there were 17,328 people, 7,385 households, and 4,645 families residing in the town. The population density was 1,776.0 people per square mile (685.5/km2). There were 7,707 housing units at an average density of 789.9 per square mile (304.9/km2).