Portsmouth is a city in Rockingham County, New Hampshire in the United States. It is the fourth-largest community in the county,[1] with a population of 20,784 at the 2000 census. A historic seaport and popular summer tourist destination, Portsmouth is served by Portsmouth International Airport at Pease, formerly the Strategic Air Command's Pease Air Force Base converted into a civilian airport and office park.

At the town's incorporation in 1653, it was named Portsmouth in honor of the colony's founder, John Mason. He had been captain of the port of Portsmouth, England, in the county of Hampshire, for which New Hampshire is named. In 1679, Portsmouth became the colonial capital. It also became a refuge for exiles from Puritan Massachusetts. When Queen Anne's War ended, the town was selected by Governor Joseph Dudley to host the 1713 Treaty of Portsmouth, which temporarily ended hostilities between Abenaki Indians and English settlements of the Province of Massachusetts Bay and New Hampshire.