Portsmouth is a city in Rockingham County, New Hampshire, United States. As of the 2010 census it had a population of 21,233, and in 2019 the estimated population was 21,927. A historic seaport and popular summer tourist destination on the Piscataqua River bordering the state of Maine, Portsmouth was formerly the home of the Strategic Air Command's Pease Air Force Base, since converted to Portsmouth International Airport at Pease.