The Flat River is a tributary of the Grand River in the western part of the U.S. state of Michigan. It rises as the outflow of the six lakes system in Belvidere Township, east of Edmore in Montcalm County, at . It flows mostly south and slightly west through Montcalm County, Ionia County, and Kent County, flowing into the Grand River in Lowell at . Other cities it flows through along the way include Greenville and Belding.
Two historic covered bridges cross the river. One, the Fallasburg Bridge, in Vergennes Township, Kent County is in Fallasburg County Park a few miles north of Lowell. The other, Whites Bridge is a few miles to the northeast in adjacent northwest Keene Township, Ionia County, near Smyrna.
It is the river named in the song "Sad Ballad of Jack Haggerty", sometimes known as "Flat River Girl."