Lehigh County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the county's population was 374,557. Its county seat is Allentown, the state's third-largest city after Philadelphia and Pittsburgh. Lehigh County and Northampton County to its east combine to form the Lehigh Valley region of eastern Pennsylvania. Lehigh County is one of the fastest-growing counties in Pennsylvania and the more highly populated of the two counties.