Chester County 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 534,413, increasing by 7.1% from 498,886 in 2010. The county seat and most populated municipality is West Chester. Chester County was one of the three original Pennsylvania counties created by William Penn in 1682. It was named for Chester, England.