M-91 is a largely north-south state highway in the Lower Peninsula of the U.S. state of Michigan. It runs from Beling to south of Lakeview. Its 24-mile length exists entirely within Ionia and Montcalm counties. It is all undivided surface route. Aside from the two Michigan state highways that are its termini, it has only one other interstection with any Michigan state highway, and that is with M-57 in Greenville, Michigan. Aside from the towns along its way it serves inland resorts used heavily by people from Grand Rapids.