county, west-central Pennsylvania, U.S., bounded to the north by the Allegheny River and Redbank Creek and to the south by the Kiskiminetas River. It consists of a hilly region on the Allegheny Plateau, through which the Allegheny River has cut a deep valley roughly north-south in the western part of the county. Other waterways include Mahoning, Buffalo, and Crooked creeks, as well as Keystone, Mahoning Creek, and Crooked Creek lakes.

The county, created in 1800, is named for John Armstrong (Armstrong, John), a military officer and diplomat who, during the French and Indian War, captured Kittanning (Sept. 8, 1756), the largest Delaware Indian town in western Pennsylvania. Kittanning is the county seat. Some other communities are Ford City, Freeport, and Apollo.

The main components of the economy are retail trade, services, manufacturing, and bituminous coal mining. Area 654 square miles (1,694 square km). Pop. (2000) 72,392; (2007 est.) 69,059.