county, southwestern Pennsylvania, U.S., bounded to the north by Jacobs Creek; to the east by Laurel Hill, the Youghiogheny River, and Youghiogheny River Lake; to the south by Maryland and West Virginia; and to the west by the Monongahela River. It consists of a hilly region on the Allegheny Plateau that rises to the Allegheny Mountains in the east (including Chestnut Ridge). Other features include Redstone and Indian creeks and parts of Ohiopyle and Laurel Ridge state parks.

The Fort Necessity National Battlefield commemorates the opening battle (July 3, 1754) of the French and Indian War, in which Colonel George Washington (Washington, George) surrendered to the French. The county was created in 1783 and named for the marquis de Lafayette (Lafayette, Marie-Joseph-Paul-Yves-Roch-Gilbert du Motier, marquis de). In 1936 architect Frank Lloyd Wright (Wright, Frank Lloyd) designed the house Fallingwater, which was built over the Bear Run waterfall in the community of Mill Run.

The primary cities are Connellsville and Uniontown, which is the county seat. Major components of the economy include tourism, health care services, manufacturing, and bituminous coal mining. Area 790 square miles (2,046 square km). Pop. (2000) 148,644; (2007 est.) 144,556.