U.S. Route 85 (US 85) is a north–south United States highway that runs for 1,479 miles (2,380 km) in the Mountain - Northern Plains states of the United States. The southern terminus of the route is at the United States-Mexico border in El Paso, Texas, connecting with Mexican Federal Highway 45. The northern terminus is at the United States-Canada border in Fortuna, North Dakota, where the route continues north as Saskatchewan Highway 35. In both the United States and Saskatchewan, Canada the route is known as the CanAm Highway. Sections of US 85 are considered part of the Theodore Roosevelt Expressway.

The highway passes through Texas, New Mexico, Colorado, Wyoming, South Dakota, and North Dakota.

US 85 in Texas begins at the United States/Mexico border with U.S. 62 and travels north through El Paso, beginning at the Santa Fe Street Bridge, and following Santa Fe street, then Paisano Drive westward, along the Rio Grande until Paisano Drive ends where it joins with I-10, about 14 miles (23 km) before both reach the New Mexico border. The route is concurrent with Interstate 10 for the remainder of its route within Texas.

The original route of US 85 in Texas had the highway concurrent with Doniphan Drive (after Paisano Drive), and parallelling the Atchison, Topeka and Santa Fe Railway through the Mesilla Valley communities of Canutillo, Vinton and Anthony before crossing the Texas/New Mexico state line in Anthony, New Mexico, then following the road which is now New Mexico State Road 478 up the Mesilla Valley to Las Cruces. This route is marked as Texas State Highway 20 north of the intersection with Mesa Street/Country Club Drive.

The unsigned route of US-85 through New Mexico exists only on paper to maintain continuity with signed sections in Colorado and Texas. Except for a 4-mile segment through Las Vegas, U.S. 85 in New Mexico is entirely concurrent with Interstate Routes. For the first 20 miles (32 km) it shares its route with I-10, then continues north for the remainder of its length in New Mexico concurrent with I-25. US-85 was de-signed in segments between 1970 and 1990 as I-25 was built through the state. I-25 between Bernalillo and a point just south of Santa Fe was built over the old US-85 alignment. I-25 was also built directly over US-85 from east of Santa Fe to Las Vegas and from US-64 to the Colorado border at Raton Pass.

The original route from Anthony to Las Cruces is now signed as New Mexico State Road 478. The original route from Las Cruces to Hatch is now signed as New Mexico State Road 185; New Mexico State Road 187 south of Truth or Consequences (T or C); New Mexico State Road 181 north of T or C; New Mexico State Road 1 from Redrock to Socorro; New Mexico State Road 314 from Belen to Albuquerque; New Mexico State Road 313 from Albuquerque to Bernalillo; New Mexico State Road 14 and New Mexico State Road 466 through Santa Fe; and New Mexico State Road 445 from Maxwell to US Route 64 Remnant US-85 signs can still be seen on Fourth Street in downtown Albuquerque, the original route through the city before I-25 was built.

The New Mexico Department of Transportation has removed US-85 from its route logs, but the American Association of State Highway and Transportation Officials (AASHTO) retains US-85 on a concurrent alignment with I-10 and I-25 to maintain continuity with signed segments in Texas and Colorado.

U.S. 85 enters Colorado from New Mexico concurrent with I-25 but is not signed. US-85 leaves I-25 at Exit 128 and follows Santa Fe Avenue through Fountain before turning west briefly onto Lake Avenue, then turning north following Nevada Avenue through Colorado Springs before rejoining I-25 at Exit 148. Approaching the south side of Denver, US-85 again leaves I-25 at Exit 182. From there it heads west and north as a two lane rural highway. It becomes an expressway near Chatfield Lake and the southern Denver suburbs of Littleton and Englewood. It continues north through Denver for a few miles before once again joining with I-25 at mile marker 207. There it becomes a concurrency with U.S. Route 87 as well as I-25 and heads north through downtown Denver. At exit 214 U.S. 85 turns east and becomes a concurrency with Interstate 70 and U.S. Route 6 for about a mile where it exits with U.S. 6 and heads northeast through Commerce City. In just a few miles the U.S. 6/U.S. 85 concurrency merges with Interstate 76 at mile marker 9. They travel concurrently for 3 miles (4.8 km) until exit 12 when U.S. 85 becomes an expressway and continues north out of the Denver area through Brighton. From there it parallels I-25 for about 75 miles (121 km) passing through Fort Lupton, Platteville, Evans, Greeley, and Eaton before crossing into Wyoming.

US 85 enters Wyoming from Colorado 8 miles (13 km) south of Cheyenne. In Cheyenne it joins with Business Route 87, and a mile later with Interstate 180 until it meets with U.S. Route 30. The segment with I-180 is the only fully at-grade interstate route in the U.S. At exit 12, it joins with I-25 and U.S. Route 87 in a concurrency for 5 miles (8.0 km) until US 85 leaves at exit 17 and travels northeast towards Meriden. From there it heads north to Torrington, where it meets with U.S. Route 26 and concurrencies for 10 miles (16 km) until Lingle, and 47 miles (76 km) later it meets U.S. Route 20 and U.S. Route 18 at Lusk. It shares the next 47 miles (76 km) with US 18 and 33 miles (53 km) later meets U.S. Route 16 near Newcastle. From here it is 29 miles (47 km) until it enters South Dakota in the Black Hills.

The South Dakota section of U.S 85., with the exception of two concurrencies with U.S. 14 Alternate and a concurrency with Interstate 90, is defined at South Dakota Codified Laws § 31-4-181.

U.S. 85 enters the Black Hills from Wyoming and travels northeast until it meets with U.S. 14 Alternate east. The two routes form a concurrency from there until I-90 where U.S. 85 overlaps for 8 miles (13 km) while heading west. At exit 10 U.S. 85 heads back north towards Belle Fourche where it crosses U.S. 212. From there it continues to North Dakota passing only through the small towns of Redig, Buffalo, and Ludlow.

U.S. 85 enters North Dakota in the southwest part of the state. The first city on its route is Bowman at the junction of U.S. Route 12. Continuing north, it passes between North Dakota's two highest points, White Butte and Black Butte. Near Amidon U.S. 85 heads east for 9 miles (14 km) before going back north along the Little Missouri National Grassland for about 125 miles (201 km). Near Belfield it junctions with Interstate 94. After forming a concurrency with North Dakota Highway 200, it eventually passes through part of the scenic Badlands, crosses the Little Missouri River and passes near the Theodore Roosevelt National Park (North Unit).

Then, at Watford City it travels west for 16 miles (26 km) where it turns back north before Alexander. U.S. 85 continues north as North Dakota 200 turns west toward Montana. South of Williston it crosses the Missouri River. A few miles later it meets with U.S. Route 2 where the two overlap for 19 miles (31 km) as an expressway, which now bypasses Williston to the northwest. After U.S. 2 heads east U.S. 85 continues north to a concurrency with North Dakota Highway 5. From there it is 7 miles (11 km) to Fortuna where U.S. 85 heads back north for its remaining 6 miles (9.7 km) to the U.S./Canada border.

What is now US 85 from El Paso, Texas to then-U.S. Route 66 (now SR 6) in Los Lunas, New Mexico (south of Albuquerque) was shown as U.S. Route 466 in an early 1925 plan for the U.S. Highway System. This north–south route was never signed in the field; instead, the route was designated as part of US 85. This proposed routing is often referred to as "US 466 (I)" to distinguish it from the later, signed US 466.