Rapid River is a 6.0-mile-long (9.7 km) river in the northwestern portion of the US state of Maine flowing from Lower Richardson Lake to Umbagog Lake. Despite its short length, it forms the end of a major portion of the Umbagog Lake and Androscoggin River watersheds. Much of this watershed is covered by (from downstream to upstream) Lower and Upper Richardson Lakes, Mooselookmeguntic Lake, Cupsuptic Lake and Rangeley Lake.