U.S. Highway 87 is a north–south United States highway (though it is signed east–west in New Mexico) that runs for 1,998 miles (3,215 km) from northern Montana to southern Texas. Most of the portion from Billings, Montana, to Raton, New Mexico, is co-signed along Interstates 90 and 25. It is also co-signed along the majority of Interstate 27 in Texas.

As of 2004, the highway's northern terminus is Havre, Montana, at U.S. Highway 2. Its southern terminus is Port Lavaca, Texas.

US 87 continues in a northwesterly direction in New Mexico, and is signed by NMDOT primarily as an east–west route. It merges with US 64 (and thus the Santa Fe Trail National Scenic Byway) in Clayton, shortly after entering New Mexico. It continues to the northwest until Des Moines, when it takes a more westerly approach to Raton. In Raton, it separates from US 64 and merges with Interstate 25 and US-85, with which it remains concurrent through Raton Pass and into Colorado, though it is unsigned on much of the concurrency.

US 87 remains concurrent with Interstate 25 throughout the states of Colorado and Wyoming, of which is a rare occurrence for a US highway to have a concurrency with an Interstate in its entirety within state boundaries. For more on this section of US 87, see Interstate 25 in Colorado.

US 87 remains concurrent with Interstate 25 northward until exit 160 east of Glenrock where it joins US Route 20/US Route 26 from Glenrock to Casper. In Casper it splits from US-20/26 and rejoins I-25 at exit 186. It then remains concurrent with I-25 until its terminus with Interstate 90. It then follows I-90 west to exit 44 where it runs up to Sheridan. A portion of US-87 has been washed out for several years along this stretch and "temporary" detour signs are posted directing US-87 traffic along Wyoming Highway 193 through Story. In Sheridan US-87 rejoins Interstate 90 into Montana.

US 87 remains concurrent with Interstate 90 westward until Billings, where it breaks off and heads north. Between Crow Agency and Billings, US 87 and I-90 are merged with US 212. It intersects with (and briefly merges with) US 12 in Roundup and continues north with a slight bend to the northwest until, at Grass Range it takes a sharp turn to the west at an intersection with Montana State Highway 200. US 87 remains concurrent with Montana State Highway 200 until Great Falls. In Lewistown, it merges with US 191 and remains heading generally west. Some ten miles (16 km) out of Lewistown, it breaks with US 191 and merges with Montana State Highway 3, heading generally northwest and merging briefly with US 89 before breaking with all three in Great Falls. US 87 heads northeast then east to Fort Benton and then generally northeast to its terminus with US 2 about two miles (3 km) west of Havre.

U.S. Route 185 was formed in 1926, and extended from US 85 in Cheyenne north to Orin. It became part of a southern extension and realignment of US 87 in 1936.