M-139 is a state trunkline highway in the US state of Michigan, entirely within Berrien County. It was formed originally as a bypass of the Benton Harbor and St. Joseph area, but now it serves to provide access into the area from a set of bypasses consisting of Interstate 94 (I-94) and US Highway 31 (US 31).

M-139 begins at a junction with US 12 southwest of Niles in Bertrand Township, traveling northeast into downtown Niles, where it turns north. It proceeds through Berrien Springs, meeting US 31/St. Joseph Valley Parkway just to the northwest. From there the route travels northwest through Royalton Township to the community of Scottdale where it turns northward at M-63. There is a junction with I-94 at its exit 28; M-139 then passes through the community of Fair Plain and enters the eastern edge of Benton Harbor. Before terminating at Main Street in Benton Harbor, the road splits into a one-way pair just north of Empire Avenue. Northbound traffic is routed along Martin Luther King Drive while southbound traffic flows on Fair Avenue.

M-139 was determined as a state trunkline in 1929 from Scottdale to Benton Harbor, but the first sections were not opened until 1931. In 1964 or 1965, US 31 was rerouted along M-139 from I-94 to Scottdale, and M-139 was truncated to I-94. On August 27, 2003, the St. Joseph Valley Parkway freeway carrying US 31 was completed from northwest of Berrien Springs (exit 15) to Napier Avenue near Benton Harbor (exit 24). The former section of M-139 replaced by US 31 was once again designated M-139, with M-139 extending further along the former US 31 to the freeway's exit 15. According to the Michigan Highways site:

In 2008, M-139 was planned to extend southerly to end at Main Street (Business US 12, BUS US 12) in Niles along the unsigned state trunkline OLD US 31 and BUS US 31, but after the City of Niles announced its plan to take over maintenance of the section of East Main Street between Front Street/BUS US 31 and Fifth Street/M-51 in order to ban trucks, the plan was revised to extend M-139 further south, replacing the BUS US 12 designation southwesterly to US 12. The extension was executed on March 5, 2010, prior to route marker installation.

The entire highway is in Berrien County.