M-66 is a north–south state trunkline highway on the Lower Peninsula (LP) of the US state of Michigan. It runs from the Indiana state line in the south to Charlevoix in the north. M-66 is the only state highway to run the north–south distance of the LP. It starts as a continuation of State Road 9 (SR 9) which provides access to the Indiana Toll Road. The total length is approximately , which includes almost of freeway between Interstate 94 (I-94) and downtown Battle Creek designated as I-194. A section of the highway immediately south of I-94 is an expressway, a type of divided limited-access highway, while the section along I-194 is a full freeway; otherwise M-66 is a two-lane rural highway. Two sections are listed on the National Highway System.