Map - Chester County, Pennsylvania (Chester County)

Chester County (Pennsylvania Dutch: Tscheschter Kaundi), colloquially known 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 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.

Chester County is part of the Philadelphia-Camden-Wilmington, PA-NJ-DE-MD Metropolitan Statistical Area. Eastern Chester County is home to many communities that comprise part of the Philadelphia Main Line western suburbs outside of Philadelphia, while part of its southernmost portion is considered suburban Wilmington, along with southwest Delaware County.

Philadelphia, Bucks, and Chester were the three Pennsylvania counties initially created by William Penn on August 24, 1682. At that time, Chester County's borders were Philadelphia County to the north, the ill-defined western edge of the colony (approximately the Susquehanna River) to the west, the Delaware River to the east, and Delaware and Maryland to the south. Chester County replaced the Pennsylvania portion of New Netherland/New York’s "Upland", which was officially eliminated when Pennsylvania was chartered on March 4, 1681, but did not cease to exist until June of that year. Much of the Welsh Tract was in eastern Chester County, and Welsh place names, given by early settlers, continue to predominate there.

The fourth county in the state, Lancaster County, was formed from Chester County on May 10, 1729. On March 11, 1752, Berks County was formed from the northern section of Chester County, as well as parts of Lancaster and Philadelphia counties.

The original Chester County seat was the City of Chester, a center of naval shipbuilding, at the eastern edge of the county. In an effort to accommodate the increased population of the western part of the county, the county seat was moved to a more central location in 1788; in order to mollify the eastern portion of the county, the village, known as Turk's Head, was renamed West Chester. In response to the new location of the county seat, the eastern portion of the county separated and formed the new Delaware County in 1789 with the City of Chester as its county seat.

Much of the history of Chester County arises from its location between Philadelphia and the Susquehanna River. The first road to "the West" (meaning Lancaster County) passed through the central part of Chester County, following the Great Valley westward; with some re-alignments, it became the Lincoln Highway and later U.S. Route 30. This road is still named Lancaster Avenue in most of the Chester County towns it runs through. The first railroad (which became the Pennsylvania Railroad) followed much the same route, and the Reading Railroad progressed up the Schuylkill River to Reading. Industry tended to concentrate along the rail lines. Easy transportation allowed workers to commute to urban jobs, and the rise of the suburbs followed. To this day, the developed areas form "fingers" extending along major lines of transportation.

During the American Revolutionary War, the Battle of Brandywine was fought at what is now the southeastern fringe of the county. The Valley Forge encampment was at the northeastern edge.