Northumberland is a town located in western Coös County, New Hampshire, United States, north of Lancaster. It is part of the Berlin, NH–VT micropolitan statistical area. As of the 2020 census, the town population was 2,126, of whom 1,068 lived in the village of Groveton.

Coös County or Coos County is a county in the U.S. state of New Hampshire. As of the 2020 census, the population was 31,268, making it the least-populated county in the state. The county seat is Lancaster.