Portsmouth is a city located in Rockingham County within the state of New Hampshire, United States. Situated on the Piscataqua River, it serves as a historic seaport that borders the state of Maine. The city functions as a significant regional hub, combining its historical maritime roots with modern administrative roles in local governance and tourism.

European settlement began on the west bank of the harbor in 1630, initially named Strawbery Banke before being incorporated as Portsmouth in 1653. The town was named in honor of John Mason, the colony's founder who had been captain of the English port of Portsmouth. Its strategic location fostered prosperity through fishing, lumber, and shipbuilding, while its role in the American Revolution and subsequent industrial shifts shaped its development.

The city is defined by its compact, walkable downtown core featuring stately brick Federalist stores and townhouses built after devastating fires in the early 19th century. Portsmouth preserves a rich architectural legacy through its Historic District Commission and notable sites like the USS Albacore submarine museum. The area retains dense blocks of Revolution-era homes, reflecting a commitment to historic preservation amidst urban renewal efforts.

As a popular summer tourist destination, Portsmouth attracts visitors with its waterfront cafes, restaurants, and shops around Market Square. The city annually celebrates this revitalization with Market Square Day, a tradition dating back to 1977. Its cultural appeal is enhanced by numerous museums housed in historic buildings and a vibrant arts scene that draws artists and tourists alike.

The population was recorded at 21,956 as of the 2020 census. The local economy has transitioned from its shipbuilding past to include tourism and aviation, notably with the Portsmouth International Airport at Pease. The city offers excellent access via its historic port and modern transport links, providing a mild climate suitable for year-round visitation.