U.S. Route 31 is a long north–south highway connecting northern Michigan to southern Alabama, with its northern terminus at Interstate 75 near Mackinaw City, Michigan, and southern terminus at the combined U.S. Route 90 & U.S. Route 98 at Spanish Fort, Alabama. This highway formerly reached Mackinaw City along the southern approaches of the Mackinac Bridge (which has been taken over by Interstate 75) in the north and downtown Mobile, Alabama in the south via a long bridge over Mobile Bay.

Within the state of Alabama from Birmingham north to Decatur, this highway is sometimes called the "Decatur Highway." From Birmingham south to Montgomery, US 31 is sometimes called the "Montgomery Highway." South of Montgomery, US 31 is sometimes called the "Mobile Highway". Markers in North Alabama also identify it as the "Beeline Highway".

In downtown Birmingham, the junction of U.S. 31 with I-20/I-59 is also the northwestern terminus of US 280. US 31 and US 280 run together along the Red Mountain Expressway Cut (the "Elton B. Stevens" Expressway), leading to the southern and southeastern suburbs of Birmingham.

Well north of Birmingham, US 31 shares the roadway with I-65 between Blount Springs (exit 289) and Hayden (exit 287) because US 31's roadway was converted back in the early 1960s to I-65 to cross several ridgelines of the southwestern Appalachian Mountains. This highway, which was ungraded to Interstate Highway standards, was the first stretch of Interstate Highway to be opened to traffic in Alabama.

US 31 and I-65 also share the roadway for 14 miles (23 km) from mile marker 354 in Athens, Alabama, to Tennessee's mile marker 1 in Ardmore. There are no highway signs confirming this in either state on this stretch, aside from the directional signs at the entrance ramps in Athens and Ardmore. However, this is shown on the official highway maps of both states. The original roadway of US 31 between the two locations still exists as State Route 251 (SR 251) in Alabama and SR 7 in Tennessee between Athens and Ardmore.

US 31 is paired with the unsigned SR 3 throughout the state.

The first mile of US 31 in Tennessee is concurrent to I-65. From that point it parallels I-65 to downtown Nashville. At Pulaski, the southern terminus of alternale Highway 31 US 31A goes northeast through Lewisburg, Chapel Hill and Nolensville before terminating back into Highway 31 in downtown Nashville. This US31-Alternate section in Marshall County and Williamson County is referred to as "Horton Highway" in honor of former Governor of Tennessee Henry Hollis Horton, whose former estate, now a Tennessee state park, lay along the route. Highway 31 continues due north through Lynnville, Columbia, Spring Hill, Franklin and Brentwood to Nashville. The route splits into U.S. Route 31E and U.S. Route 31W in Nashville.

U.S. Route 31E stays east of Interstate 65 in Kentucky, passing through Glasgow and Bardstown. It enters Louisville from the southeast, where it is named Bardstown Road for most of its length in the city.

U.S. Route 31W closely follows Interstate 65 from the Tennessee border, through Bowling Green, to Elizabethtown. From here it diverges to the west and passes through Fort Knox. North of Fort Knox, US 31W joins with U.S. Route 60 and roughly follows the Ohio River into Louisville. In southwestern Louisville, US 31W is called Dixie Highway. It is the major commercial street in this area.

U.S. Route 31E and U.S. Route 31W rejoin in Downtown Louisville at Second Street, and immediately cross the Ohio River on the Clark Memorial Bridge.

US 31 overlaps Interstate 65 in Jeffersonville, Indiana, after having crossed the Ohio River from Kentucky. Next, it splits off north of Jeffersonville at Clarksville and proceeds to Indianapolis. At Interstate 465 on the south side of Indianapolis, US 31 is routed onto I-465 on the east side of the city. US 31 exits I-465 (coincidentally at Exit 31) in extreme southern Hamilton County, and continues northward. US 31 then passes through or near Kokomo, Rochester, Plymouth, and Lakeville. As US 31 approaches southern South Bend, Indiana it converges with US 20 and proceeds westwards then north on a freeway (the St. Joseph Valley Parkway), bypassing South Bend and proceeding into Michigan.

In the 1930s, US 31 continued at Saint Ignace across the Straits of Mackinac, and junctioned with U.S. Route 2 (Mackinac Trail and what later became M-123).

Before the Interstate era, US 31 was a major north–south highway. Interstate 65 supplants US 31 and either 31W or 31E as a through route between Indianapolis and Mobile. South of Indianapolis, all segments of US 31 not coinciding with Interstate 65 have been reduced to roads largely of local use. Interstate 196 now carries the route of US 31 between Holland, Michigan, and St. Joseph, Michigan. All of US 31 between Ludington, Michigan, and Indianapolis, Indiana, is divided highway—some of it is freeway, including a bypass route of South Bend, Indiana. The segment between South Bend and Indianapolis is scheduled for upgrade or replacement with Interstate-standard freeway (see U.S. Route 31 in Indiana). Environmental impact studies have shown that the fifth phase of the then-to-be-constructed freeway segment between Napier Avenue near Benton Harbor, Michigan, and I-94 may not be complete until 2015 (see St. Joseph Valley Parkway).

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.

At its southern end, US 31 originally went further west than it currently does, passing through Spanish Fort, Alabama. It crossed Mobile Bay via several narrow bridges, including the Admiral Raphael Semmes Bridge, a drawbridge spanning the Tensaw River. It turned north along the east bank of the Mobile River, crossing the river into Plateau, Alabama, over the Cochrane Bridge (removed), another old drawbridge where the current Cochrane–Africatown USA Bridge is now located. It then turned south to end at U.S. Route 90. It now officially ends at U.S. Route 98 in Spanish Fort, Alabama.

US 31 has three planned relocation sections in Northern Indiana. See U.S. Route 31 in Indiana.