It flows approximately 4 km (2 mi).

[2] The river is formed in Hopkinton by the confluence of Parmenter Brook and the Green Fall River, about 500 feet north of Interstate 95, and just a few feet east of the Connecticut state line.

From there, it flows south-southwest, briefly entering North Stonington, Connecticut for about 500 feet, before turning southeast and crossing back into Rhode Island and continuing through Hopkinton to its mouth at the Pawcatuck River.

The Ashaway River has no named tributaries, though it has many unnamed streams that feed it.

This article related to a river in Rhode Island is a stub.