M-21 (Michigan highway)

M-21
M-21 is highlighted in red
Route information
Maintained by MDOT
Length	99.322 mi[1] (159.843 km)
Existed	c. July 1, 1919[2]–present
Major junctions
West end	M-37 in Grand Rapids
Major intersections	I-96 in Grand Rapids M-66 in Ionia US 127 near St. Johns M-52 in Owosso M-13 near Lennon I-75 / US 23 near Flint
East end	I-475 in Flint
Location
Country	United States
State	Michigan
Counties	Kent, Ionia, Clinton, Shiawassee, Genesee
Highway system
Michigan State Trunkline Highway System Interstate US State Byways
← M-20 M-21, M-21A → M-22 ← M-56 M-56 → M-57

M-21 is an east–west state trunkline highway in the US state of Michigan connecting the cities of Grand Rapids and Flint. The highway passes through rural farming country and several small towns along its course through the Lower Peninsula. Following the course of a handful of rivers, M-21 also connects some of the state's freeways like Interstate 96 (I-96), US Highway 127 (US 127) and I-75. The highway is used by between 1,700 and 36,000 vehicles daily.

M-21 was designated along the highway by July 1, 1919 between Ionia and Goodells near Port Huron. Changes made in the 1920s extended it on the west end to Holland and on the east end to Port Huron. M-21 was truncated at both of its current termini as two Interstate freeways were completed. I-196 functionally replaced M-21 between Holland and Grand Rapids with a portion retained under state maintenance as Old M-21, now M-121. I-69 replaced M-21 from Flint to Port Huron. A section of M-21 through Flint became M-56. That designation was decommissioned in 1984 when M-21 was returned to its former routing in Flint.