Pittsburgh is a city in and the county seat of Allegheny County, Pennsylvania, United States. It is the second-most populous city in Pennsylvania, after Philadelphia, and the 68th-most populous city in the U.S. The city is located in southwestern Pennsylvania at the confluence of the Allegheny River and the Monongahela River, which combine to form the Ohio River. It anchors the Pittsburgh metropolitan area, which is the largest metro area in both the Ohio Valley and Appalachia, the second-largest in Pennsylvania, and the 26th-largest in the U.S. Pittsburgh is the principal city of the greater Pittsburgh–Weirton–Steubenville combined statistical area which includes parts of Ohio and West Virginia.