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. Coös County is part of the Berlin, NH–VT Micropolitan Statistical Area. It is the only New Hampshire county on the Canada–United States border, south of the province of Quebec, and thus is home to New Hampshire's only international port of entry, the Pittsburg–Chartierville Border Crossing.