U.S. Route 285 is a north–south United States highway, running 845 miles (1,260 km) through the states of Texas, New Mexico and Colorado. The highway's southern terminus is in Sanderson, Texas at an intersection with U.S. Route 90. US 285 has always had an endpoint in Denver, Colorado, though what is now the northern terminus was once the route's southern end, and the former portion north of Denver now does not exist. The highway's northern terminus is in Denver, Colorado, at exit 201 on Interstate 25. Trucking makes up a large portion of the route's traffic, but along much of its route the road is also used for local travel from one town to the next.

The route is a secondary route of US 85, though its parent route has been largely replaced by Interstate 25. It crosses a sibling route, US 385 in Fort Stockton, Texas.

The northern section of the route, from Santa Fe, New Mexico to Denver, Colorado traverses mountainous and rocky terrain; with that in mind, anyone using the road should check weather conditions during the winter months.

As of January 2007 the road is under construction between Española and Santa Fe, New Mexico, as part of a long term project to upgrade that section of the highway to a limited access thoroughfare. By the end of 2012 this process appears to be complete.

The southern terminus of US 285 is at US 90 in Sanderson. Proceeding north from there, it crosses I-10 at Fort Stockton, and then meets I-20 at Pecos on its way to New Mexico.

As 285 traverses north on the eastern plains of New Mexico, it passes through Carlsbad and then Roswell. In Roswell, the route intersects with U.S. Route 70 and U.S. Route 380. The route next heads northwest to Vaughn where it has a brief concurrency with US Route 54 and US Route 60. The route then continues northwest and has a junction with Interstate 40 at Clines Corners.

Heading north out of Clines Corners, the route continues towards the state capital. At the outskirts of Santa Fe, the route becomes concurrent with I-25 and its parent route (US Route 85) for several miles heading west through the foothills of the Sangre De Cristo Mountains to Santa Fe. After exiting I-25, the route becomes Saint Francis Drive in Santa Fe, and becomes concurrent with US Route 84. The route continues north by northwest to Española and Chamita, where the concurrency with US 84 ends. The route then traverses the Carson National Forest where 285 now makes a long climb up to the Colorado Plateau, passing though Ojo Caliente as it ascends to the San Luis Valley. After crossing US 64, the highway passes through the village of Tres Piedras, New Mexico at the south end of the valley, then proceeds north to the Colorado border.

Heading north from the Colorado border, US 285 passes through the main part of the San Luis Valley, eventually reaching Alamosa. As the highway heads north, it begins to ascend to the northern end of the valley and eventually climbs over Poncha Pass, elevation 9,012 feet (2,747 m), and drops sharply down the other side into the Arkansas River Valley.

The highway brushes Salida and follows the Arkansas River north up the valley, then takes a sharp eastward turn just before the small town of Buena Vista. 285 then climbs over Trout Creek Pass, elevation 9,346 feet (2,849 m), and enters the high-altitude South Park basin.

A few miles north in the South Park, the route of the original narrow gauge transcontinental railroad joins US 285 in Como. The highway passes through Fairplay and the historic South Park City site, then leaves the South Park basin as climbs over Kenosha Pass, elevation 10,001 feet (3,048 m), and skirts the south side of the Mount Evans massif as it descends its way through the foothills range towards Denver.

As the highway leaves the Rocky Mountains and reaches Denver's southwest suburbs, it becomes Hampden Avenue, an important artery in the Denver metro area, then reaches its northern terminus at I-25.

On March 14, 2008 both houses of the Colorado legislature, in a unanimous vote, named the section between Kenosha Pass and C-470 the "Ralph Carr Memorial Highway."

The short piece between US 50 at Salida and US 24 at Buena Vista was originally U.S. Route 650, designated in 1926. US 285 was commissioned in 1936 along its present extent from Sanderson to Denver, mostly replacing state-numbered highways.

Between Denver and Como, US 285 mostly follows the route of the original narrow gauge transcontinental railroad, skirting the south side of the Mount Evans massif, then descending into and crossing the South Park. Como in Colorado's South Park still houses one of the few remaining narrow gauge roundhouses. The transcontinental railroad route breaks away from US 285 at Como, going northwest over Boreas Pass en route to Breckenridge, the historically rich gold fields of Leadville, and eventually connecting to California.