Northumberland is a town located in western Coös County, New Hampshire, United States, north of Lancaster.

North of the mountain ridge known as Cape Horn, near the Connecticut River, are the remains of Fort Wentworth, built by the New Hampshire Militia in 1755 during the French and Indian War.

The town was granted as "Stonington" in 1761 to John Hogg and others by Governor Benning Wentworth, and first settled in 1767 by Thomas Burnside and Daniel Spaulding.

This was formerly the junction of the Grand Trunk Railway and the Boston, Concord & Montreal Railroad—a major point of access for the northern White Mountains.

[5] The area was once known for its large corn and potato crop, starch mills, and manufacturing (leather, shoe pegs).

The Upper Ammonoosuc River runs through town in a southwesterly direction to the Connecticut and is crossed by a covered bridge at Groveton.

Cape Horn State Forest is located in the central portion of the town boundaries.