M-139 is a state trunkline highway entirely within Berrien County in the US state of Michigan. The highway starts at US Highway 12 (US 12) southwest of Niles and runs through rural areas of the county to terminate at an intersection with Business Loop Interstate 94 (BL I-94) in Benton Harbor. The highway run parallel to, and crosses over, the St. Joseph River several times as it follows a set of roads previously used for US 31 in the area.