The Oswego River (/ɒsˈwiːɡoʊ/) is a river in upstate New York in the United States. It is the second-largest river (after the Niagara River) flowing into Lake Ontario. James Fenimore Cooper’s novel The Pathfinder, or The Inland Sea is set in the Oswego River valley.

The name Oswego is a Mohawk name that means "flowing out", or specifically, "small water flowing into that which is large".

James Fenimore Cooper described the Oswego in these words:

The Oswego is formed by the junction of the Oneida and the Onondaga, both of which flow from lakes; and it pursues its way, through a gently undulating country, some eight or ten miles, until it reaches the margin of a sort of natural terrace, down which it tumbles some ten or fifteen feet, to another level, across which it glides with the silent, stealthy progress of deep water, until it throws its tribute into the broad receptacle of the Ontario.

The Oswego River starts at the confluence of the Oneida River (flowing from Oneida Lake) and the Seneca River (flowing from Seneca Lake, Cayuga Lake, and Montezuma Marsh). The river drains an area of 5,122 square miles (13,266 km), as large as the states of Rhode Island and Delaware together, comprising most of the Finger Lakes region of upstate New York.

At its mouth at Lake Ontario, the river divides the City of Oswego, just as it divides the City of Fulton 11 miles upstream.

Part of its length the Oswego Canal was built. The Oswego River also serves as a part of the New York State Canal System, providing a route from the Erie Canal to Lake Ontario. This section of the canal was completed in 1827, two years after completion of the Erie Canal. In 1917, as part of a general overhaul of the canal system, the Oswego Canal was deepened and refurbished. The canal is now 14 feet (4.3 m) deep and has an overhead clearance of 20 feet (6.1 m).

The Oswego River was listed as a Great Lakes Areas of Concern in The Great Lakes Water Quality Agreement between the United States and Canada until it was formally removed on July 21, 2006.

The river is known for its steelhead run in the early spring, followed by a salmon run in early autumn. The river is stocked annually by the New York State Department of Environmental Conservation with 140,000 Chinook salmon and 20,000 steelhead.

• List of New York rivers

• Information and Boater's Guide to the Oswego River and NYS Canal
• New York State Barge Canal information
• Where to Fish, Oswego County
• Poem that references the river by name twice.