M-155 was a signed state trunkline in the US state of Michigan that served as a spur route from Business Loop Interstate 96 (BL I-96) in Howell in Livingston County to the Hillcrest Center, the former Howell State Hospital. It was originally designated in the early 1930s and outlasted the hospital which closed in 1982. The roadway was removed from state maps after 1991 but remains under state maintenance as an unsigned trunkline listed as either M-155 or Old M-155.