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. The two-syllable pronunciation is sometimes indicated with a diaeresis, notably in the Lancaster-based weekly newspaper The Coös County Democrat and on some county-owned vehicles. The county government uses both spellings interchangeably.