The Upper Ammonoosuc River is a tributary of the Connecticut River that flows through Coös County in northern part of the northeastern U.S. state of New Hampshire. Despite its name, the river is not an upstream portion of the Ammonoosuc River, but instead a separate tributary of the Connecticut River flowing from 20 to 60 miles (32 to 97 km) to the north of the Ammonoosuc.
The Upper Ammonoosuc rises in Pond of Safety in the town of Randolph, runs first generally north through rural portions of Berlin (where it flows through the Godfrey Dam), Milan and a corner of Dummer, then west through Stark and then Northumberland where it drains into the Connecticut near the village of Groveton. The end points of that 42.4-mile (68.2 km) course are approximately 25 air miles (38 km) apart.
From Milan to Groveton it is fairly closely paralleled by New Hampshire Route 110.
Fort Wentworth was built in 1755 at the junction of the Upper Ammonoosuc and the Connecticut Rivers (now near the village of Groveton). The fort was used sporadically during both the French and Indian War and the American Revolutionary War.