Coös County (with two syllables), usually spelled simply Coos County, is a county in the U.S. state of New Hampshire, including the whole of the state's northern panhandle. The two-syllable pronunciation is sometimes indicated with a dieresis, notably in the Lancaster-based weekly newspaper The Coös County Democrat and on some county-owned vehicles.
Coös occupies the largest area of any New Hampshire county, but has the smallest population: 33,055, as of 2010. It is the only New Hampshire county to have lost population between the 2000 and 2010 censuses. The county seat is Lancaster. Major industries are forestry and tourism, with the once-dominant paper-making industry in sharp decline.
Coös County was separated from the northern part of Grafton County, New Hampshire and organized at Berlin December 24, 1803, although the county seat was later moved to Lancaster, with an additional shire town at Colebrook. The name Coös derives from the Algonquian Indian term meaning crooked, the Indian name of the Connecticut River, which rises in the northernmost end of the county.
During the American Revolutionary War two units of troops of the Continental Army — Bedel's Regiment and Whitcomb's Rangers — were raised from the settlers of Coös. From the Treaty of Paris of 1783 until 1835 the boundaries in the northern tip of the county (and New Hampshire itself) were disputed with Lower Canada (which was soon to become part of the Province of Canada), and for some years residents of the area formed the independent Republic of Indian Stream.
In the 1810 census there were 3,991 residents, and by 1870 there were nearly 15,000, at which point the entire county was valued at just under $USD 5 million, with farm productivity per acre comparing favorably with that of contemporary Illinois. Other early industries included forestry and manufacturing, using 4,450 water horsepower in 1870.