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. Along with Northampton County to its east, the two counties combine to form the Lehigh Valley region of eastern Pennsylvania, the third-largest metropolitan area of Pennsylvania with a population of 861,889 as of 2020.