Pittsburgh is a city in the Commonwealth of Pennsylvania in the United States, and is the county seat of Allegheny County. As of 2017, a total population of 305,704 lives within the city limits, making it the 63rd-largest city in the U.S. The metropolitan population of 2,353,045 is the largest in both the Ohio Valley and Appalachia, the second-largest in Pennsylvania (behind Philadelphia), and the 26th-largest in the U.S.