county, south-central New York state, U.S., bordered by Pennsylvania to the south. It consists of a hilly region drained by the Chemung River (which bisects the county roughly north-south) and its tributaries. Forested areas feature a variety of hardwoods.

On Aug. 29, 1779, the American Revolutionary War generals John Sullivan (Sullivan, John) and James Clinton defeated a force of Seneca Indians and British Tories in the Battle of Newtown along the Chemung River. Elmira, the county seat, is known for the Elmira Reformatory (opened 1876), a pioneer of prison reform, and Elmira College (founded 1855), one of the nation's earliest institutions of higher learning for women. The Woodlawn National Cemetery contains the graves of nearly 3,000 Confederate prisoners and of writer Mark Twain (Twain, Mark), who summered in the county in his later years.

The county was created in 1836 and named for a Delaware Indian village. Local industry was spurred by the arrival of the Erie Railroad (Erie Railroad Company) in 1849. Among the other communities are Elmira Heights, Horseheads, Big Flats, and Southport. The county's economy is based on services, manufacturing, and retail trade. Area 408 square miles (1,057 square km). Pop. (2000) 91,070; (2007 est.) 88,015.