US Highway 31 (US 31) is a part of the United States Numbered Highway System that runs from Alabama, to the Upper Peninsula of the US state of Michigan. In Michigan, it is a state trunkline highway that runs from the Indiana–Michigan state line at Bertrand Township to its terminus at Interstate 75 (I-75) south of Mackinaw City. The highway runs for 357.234 miles (574.912 km) in the state.

The route crossed the Straits of Mackinac for a time before the Mackinac Bridge was built by ferry only.

Much of US 31 from the Indiana-Michigan state line to Ludington is built to freeway standards. Two notable exceptions are a short segment along Napier Avenue between the St. Joseph Valley Parkway and I-94 near Benton Harbor, and between Holland and Ferrysburg. The portion of I-75 north of US 31's northern terminus toward the Mackinac Bridge was designated US 31 prior to the 1990s and was US 31's most northern portion to be built to freeway status. Since its truncation at the I-75 junction, however, the most northern segment of US 31 freeway ends near Ludington.

The southernmost 24 miles (39 km) of US 31 in Michigan runs along the four-lane freeway St. Joseph Valley Parkway. The portion of US 31 concurrent with I-94 near Benton Harbor travels along a six-lane freeway, while the next 47 miles (44 of which are concurrent with I-196) from north of Benton Harbor to Holland a four-lane freeway. The 24 miles (39 km) from Holland to Grand Haven runs along a mixture of four-lane divided highway and four-lane divided expressway. At Grand Haven, US 31 crosses the Grand River via a six-lane bascule bridge. The next 67 miles (108 km) from Ferrysburg to Ludington travels along a four-lane freeway. From Ludington to its northern terminus south of Mackinaw City, US 31 consists largely of two-lane rural highway and four- to five-lane urban street segments. Two segments, however, have been constructed to two-lane limited-access standards: from Elk Rapids northerly and from Carp Lake to I-75.

At the introduction of the US Highway System in the 1920s the route of US 31 largely replaced the existing M-58 and M-11 route designations.

In the 1920s and 30s, US 31 crossed the Straits of Mackinac via auto ferry and terminated at US 2 in Rogers Park north of St. Ignace. The former US 31/US 33, now M-51, enters the state of Michigan from Indiana in Niles Charter Township.

The Michigan Memorial Highway Act (Act 142 of 2001) designates all of US 31 from the Indiana border to the Straits of Mackinac as the Blue Star Memorial Highway. "Act 71 of 1952" had previously designated all of US 31 in the Lower Peninsula and the portion of US 2 from St. Ignace to Sault Ste. Marie, although this was not included in the Act of 2001. Various portions of former routings of US 31 throughout southwest and west Michigan, notably County Road A-2, are still called "Blue Star Highway".

MDOT maintains a listing of the historic bridges in the state; along US 31, the department has listed four structures.

In downtown Charlevoix, the US-31–Island Lake Outlet Bridge carries the highway over a channel dredged between Lake Michigan and Round Lake. Built from 1947 through 1949, it is the fifth bridge at the location. It is a double-leaf bascule bridge.

In Petoskey, the highway crosses Bear Creek on a concrete girder bridge built in 1930. At 265 feet (81 m) in length, it is the longest such bridge in Michigan.

In Manistee, the Manistee River is spanned by a double-leaf bascule bridge built in 1933.

North of Hart in Pentwater Township, the 270-foot-long (82 m) US 31–Pentwater River Bridge is a long-span steel bridge that crosses the Pentwater River. It was listed on the National Register of Historic Places on December 20, 1999. It carries Oceana Drive along a former routing of US 31.

According to the recently passed Federal Transportation Bill an earmark was provided to begin planning and right of way purchases for a new route of the highway along the 120th ave corridor in Ottawa County, between Holland and Nunica. Due to tight transportation budgets at the state level, the Michigan Department of Transportation will begin construction on this project using a staged approach. The first phase in Ottawa County, designated Future M-231, will consist of a two-lane highway constructed on four-lane right-of-way for future conversion to freeway standards from M-45 (Lake Michigan Dr) to I-96 at Nunica. This segment will include a new crossing over the Grand River. MDOT is currently working on environmental clearances for this project with right-of-way acquisition to begin thereafter. Any work on the proposed US 31 freeway corridor south of M-45 (Lake Michigan Dr) has not yet been scheduled.

Plans also call for the completion of the St. Joseph Valley Parkway, to carry US 31, between Napier Avenue and I-94 sometime in the 2010s.