Allentown is a city located in Lehigh County, Pennsylvania, United States. It is Pennsylvania's third most populous city and the 231st largest city in the United States. As of the 2010 census, the city had a total population of 118,032 and is currently the fastest growing city in all of Pennsylvania. It is the largest city in the metropolitan area known as the Lehigh Valley, which had a population of 821,623 residents as of 2010. Allentown constitutes a portion of the New York City Combined Statistical Area and is the county seat of Lehigh County. In 2012, the city celebrated the 250th anniversary of its founding in 1762.Located on the Lehigh River, Allentown is the largest of three adjacent cities, in Northampton and Lehigh counties, that make up a region of eastern Pennsylvania known as the Lehigh Valley.