Coös County (with two syllables), frequently spelled Coos County, is a county in the U.S. state of New Hampshire. As of the 2010 census, the population was 33,055, the least of any New Hampshire county. The county seat is Lancaster.
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. The county government uses both spellings interchangeably.
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.
Coös County includes the whole of the state's northern panhandle. Major industries include forestry and tourism, with the once-dominant paper-making industry in sharp decline. The county straddles two of the state's tourism regions. The southernmost portion of the county is part of the White Mountains Region and is home to Mount Washington. The remainder of the county is known as the Great North Woods Region.
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 word meaning "small pines".
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.