The Allegheny River ( /ælɨˈɡeɪni/) is a principal tributary of the Ohio River; it is located in the Eastern United States. The Allegheny River joins with the Monongahela River to form the Ohio River at the "Point" of Point State Park in Downtown Pittsburgh, Pennsylvania. The Allegheny River is, by volume, the main headstream of the Ohio River.

The river is approximately 325 miles (523 km) long, running through the U.S. states of New York and Pennsylvania. It drains a rural dissected plateau of 11,580 square miles (30,000 km²) in the northern Allegheny Plateau, providing the northeastern most drainage in the watershed of the Mississippi River. Its tributaries reach to within 8 miles (13 km) of Lake Erie in southwestern New York.

The Allegheny Valley has been one of the most productive areas of fossil fuel extraction in U.S. history, with its extensive deposits of coal, petroleum, and natural gas.

The name Allegheny probably comes from Lenape welhik hane or oolikhanna, which means 'best flowing river of the hills' or 'beautiful stream'. There is a Lenape legend of a tribe called "Allegewi" who used to live along the river.