Allentown is a city in the Commonwealth of Pennsylvania and the county seat of Lehigh County, in the United States. It is the third-most-populous city in Pennsylvania with a population of 125,845 as of the 2020 census and the largest city in the Lehigh Valley metropolitan area, which had a population of 861,899 and was the 68th-most populous metropolitan area in the nation as of 2020.Founded in 1762, Allentown is located on the Lehigh River, a 109-mile-long tributary of the Delaware River.