county, northern Pennsylvania, U.S., bounded to the north by New York state. It consists of a mountainous region on the Allegheny Plateau that is drained largely by the Allegheny River and such tributaries as Kinzua, Marvin, Potato, and Allegheny Portage creeks. Parklands include Kinzua Bridge State Park and part of Allegheny National Forest.

The county was formed in 1804 and named for Thomas McKean, then governor of Pennsylvania. Land prices skyrocketed in Bradford after oil was discovered there in 1875. Other communities include Kane, Port Allegany, and Smethport, which is the county seat. The main economic activities are manufacturing, services, retail trade, and mining. Area 981 square miles (2,542 square km). Pop. (2000) 45,936; (2007 est.) 43,633.