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 First Lake, the last in a chain of geographically close, connected lakes known as Six Lakes in Belvidere Township, west of Edmore in Montcalm County. It flows mostly south and slightly west through Montcalm County, Ionia County, and Kent County, flowing into the Grand River in Lowell. Other cities it flows through along the way include Greenville and Belding.

The Grand River is a 252-mile-long (406 km) river in the Lower Peninsula of the U.S. state of Michigan. The longest river in Michigan, the Grand River rises in Hillsdale County, and flows in a generally northwesterly direction to its mouth at Lake Michigan in the city of Grand Haven. The river flows through a number of cities, including Jackson, Lansing, Ionia, and Grand Rapids.