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.

New Hampshire is a state in the New England region of the Northeastern United States. It borders Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec to the north. Of the 50 U.S. states, New Hampshire is the fifth smallest by area and the tenth least populous, with a population of 1,377,529 residents as of the 2020 census. Concord is the state capital and Manchester is the most populous city. New Hampshire's motto, "Live Free or Die", reflects its role in the American Revolutionary War; its nickname, "The Granite State", refers to its extensive granite formations and quarries. It is well known nationwide for holding the first primary in the U.S. presidential election cycle, and for its resulting influence on American electoral politics.