Chester County (Pennsylvania Dutch: Tscheschter Kaundi), 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. The most populous boroughs are West Chester and Phoenixville. Coatesville is the only municipality in the county that is classified as a city. The county is part of the Southeast region of the commonwealth.

Chester County was one of the three original Pennsylvania counties created by William Penn in 1682. It was named for Chester, England. It is part of the Philadelphia-Camden-Wilmington, PA-NJ-DE-MD metropolitan statistical area. Along with northern Delaware County and southern Montgomery County, eastern Chester County is home to many communities that comprise part of the Philadelphia Main Line western suburbs of Philadelphia.

As of 2020, the county had the highest median household income level in Pennsylvania, and the 35th-highest in the nation.

Philadelphia, Bucks, and Chester were the three counties created by William Penn on August 24, 1682, in the colonial-era Province of Pennsylvania.

At the time, Chester County's borders were Philadelphia County to the north, the ill-defined western edge of the colony, located approximately at 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's upland in New York, which was officially eliminated when Pennsylvania was chartered on March 4, 1681, and ceased existing in 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 and parts of Lancaster and Philadelphia counties.

The southern border of Chester County is the Mason-Dixon Line, surveyed in 1765. An error in surveying resulted in the Wedge, extending south of the line. Chester County claimed the Wedge until 1921, when it was ceded to Delaware.^{[citation needed]}

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.