Chester is a city in Delaware County, Pennsylvania, United States. It is located in the Philadelphia metropolitan area (also known as the Delaware Valley) on the western bank of the Delaware River between Philadelphia and Wilmington, Delaware. The population of Chester was 32,605 at the 2020 census.

Incorporated in 1682, Chester is the oldest city in Pennsylvania and was the location of William Penn's first arrival in the Province of Pennsylvania. It was the county seat for Chester County from 1682 to 1788 and of Delaware County from 1789 to 1851.

From the second half of the 19th century through the first half of the 20th century, the city was a major center of heavy industry, manufacturing and shipping. The city became a boomtown during World War I and World War II. The availability of employment in factories, dock work, and shipbuilding attracted immigrants from Southern and Eastern Europe and African-American migrants from southern states. Since the mid-20th century, it has lost most of its manufacturing base and has struggled as a post-industrial city dealing with crime, pollution, and poverty. Pennsylvania declared Chester a financially distressed municipality in 1995 and a fiscal emergency in 2020. In 2022, Chester became the 31st city in the United States to declare bankruptcy.

The city is home to Widener University, Harrah's Philadelphia, the Chester Waterside Station, the William Penn Landing Site, and Subaru Park, home of the Major League Soccer team, the Philadelphia Union.