The Governor John Langdon House, also known as Governor John Langdon Mansion, in Portsmouth, New Hampshire, USA, is a mansion house built in 1784 by John Langdon, that is now a National Historic Landmark. Langdon was a merchant, shipbuilder, American Revolutionary War general, signer of the United States Constitution, and three-term President (now termed governor) of New Hampshire. The house he built for his family expresses his status as Portsmouth's leading citizen and received praise from George Washington, who visited there in 1789. Its reception rooms are ornamented by elaborate wood carving in the rococo style.
After Langdon's death in 1819, the house was occupied by other leading families. At the end of the 19th century, Langdon descendants purchased the house and restored it to its 18th-century glory, adding on a substantial wing designed by McKim, Mead, and White to house modern conveniences. The garden, dating from the same era, features restored perennial beds, a rose and grape arbor, and a pavilion. The house and grounds are now owned and operated as a house museum by Historic New England. The house was declared a National Historic Landmark in 1974.