county, southwestern Pennsylvania, U.S., consisting of a hilly region on the Allegheny Plateau bounded to the southeast by the Monongahela (Monongahela River) and Youghiogheny (Youghiogheny River) rivers and to the northeast by the Allegheny River. The Ohio (Ohio River), Allegheny, and Monongahela rivers converge in the centre of the county to form an area known as the Golden Triangle; this was a strategic point of contention between the French and the English, who fortified the area with Fort Duquesne (1754) and Fort Pitt (1761), respectively. With the defeat of Pontiac's warriors (1763), the area opened up to settlers who founded Pittsburgh (1764), which became the county seat when Allegheny county was formed in 1788. The county's name is derived from the Delaware Indian word oolikhanna, meaning “good river.” The Triangle is now Pittsburgh's central business district and the location of the popular Point State Park.

Pittsburgh, which was linked to outside markets by the Pennsylvania Canal and the Portage Railroad (both completed in 1834), became the nation's centre for the iron, steel, and glass industries in the 19th century. The city is also the home of the University of Pittsburgh (Pittsburgh, University of) (1787), Duquesne University (1878), and Carnegie Mellon University (1900), as well as the museums, library, and music hall of The Carnegie, formerly the Carnegie Institute.

Other communities include Bethel Park, Monroeville, McKeesport, Baldwin, and Wilkinsburg. The main economic activities are services (health care, business, education, and engineering), retail trade, manufacturing (steel and industrial machinery), and finance. Area 730 square miles (1,891 square km). Pop. (2000) 1,281,666; (2007 est.) 1,219,210.