county, west-central New York state, U.S., comprising a hilly upland region bounded by Canandaigua Lake to the northwest, Keuka Lake to the south, and Seneca Lake to the east. Other waterways are the West River and Flint Creek. State lands include Keuka Lake State Park and High Tor Wildlife Management Area. Bluff Point is an elevated region that divides the west and main branches of Keuka Lake. Forests mainly consist of a mix of hardwoods.

In 1779 Seneca Indians of the Iroquois Confederacy were defeated by an American military expedition to the region led by U.S. Major General John Sullivan (Sullivan, John). The county was formed in 1823 and named for Joseph Yates, then governor of New York. The county seat was named Penn Yan because it was settled both by residents of Pennsylvania and by Yankees from New England. Other towns are Dundee and Keuka Park, home of Keuka College (founded 1890).

The main economic activities are tourism and agriculture (especially wine grapes). Area 338 square miles (876 square km). Pop. (1990) 22,810; (1996 est.) 24,300.