Chester County, colloquially referred to as Chesco, is a county in the Commonwealth of Pennsylvania. It is located in the Delaware Valley region, located in the southeastern part of the state. As of the 2020 census, the population was 545,823. increasing by 7.1% from 498,886 in 2010. The county seat is West Chester. The most populous of the county's 73 municipalities, including cities, boroughs, and townships,) is Tredyffrin Township.