State Highway 119 is a 63.7 mi (102.5 km) long state highway in north central Colorado. It extends in a southwest to northeast direction, from a junction with US Route 6 in Clear Creek Canyon between Golden and Idaho Springs to a junction with I-25 east of Longmont. The southwest portion of the road is a scenic mountain drive providing dramatic vistas of the Front Range, while the northeast portion is a busy interurban thoroughfare. The city of Boulder separates these two vastly different sections of Route 119.

Highway 119 begins at a signalized T intersection with US Route 6 in Clear Creek Canyon between Golden and Idaho Springs. It then heads north, alongside North Clear Creek, to the gambling town of Blackhawk; this section of the road carries heavy traffic to the casinos of Blackhawk and Central City and experiences many accidents.

It continues north from Blackhawk as the Peak to Peak Scenic and Historic Byway, skirting the western edge of Golden Gate Canyon State Park where it intersects Colorado State Highway 46. It continues north through the village of Gilpin Center to Rollinsville, where it crosses over South Boulder Creek and the tracks of the Union Pacific Railroad, on its way from Denver to the Moffat Tunnel. East Portal Road in Rollinsville provides access to the east portal of the railroad tunnel, and to the Moffat Road historic railroad grade across Rollins Pass. Route 119 continues northward, intersecting Colorado State Highway 72 with which it will be concurrent to Nederland. It provides access to the Eldora Mountain Resort ski area, about 3 miles (4.8 km) west of Nederland. In Nederland, routes 119 and 72 diverge at a roundabout intersection where Route 72 takes the Peak to Peak Highway designation with it to the north, and Route 119 turns to the east. Leaving Nederland, Route 119 passes along the northern shore of Barker Meadow Reservoir and enters the scenic Boulder Canyon, alongside Boulder Creek. In the canyon, the road experiences grades of up to 10%, which can be dangerous for heavy trucks heading downhill, as it does not have a runaway truck ramp. The walls of the canyon are popular for rock climbing, and the scenic Boulder Falls is an easy walk from the highway.

At the mouth of the canyon, Route 119 suddenly enters Boulder as Canyon Boulevard, intersecting Colorado State Highway 93 at Broadway, and turning north on 28th Street where it begins concurrency with US Route 36 and with unsigned Colorado State Highway 7. This busy section of State 119 and US 36 on 28th Street serves Boulder's main shopping area. The overlap with US 36 ends at the northern edge of Boulder where US 36 continues north to Rocky Mountain National Park, and Route 119 turns east becoming Diagonal Highway. At an intersection with Colorado State Highway 157, Route 119 bends northeast, paralleling the tracks of the Burlington Northern Santa Fe Railway, and becomes an expressway with a very wide median, possibly intended to accommodate a future freeway. At Niwot it passes a very large facility for the IBM Corporation at an intersection with Colorado State Highway 52. The Diagonal Highway expressway ends in Longmont, as Route 119 becomes Ken Pratt Blvd. and intersects US Route 287. East of US 287 in Longmont, a new expressway carries Route 119 directly to its northeastern terminus at a diamond interchange at Interstate 25 exit 240.

Route 119 was established in the 1920s, extending from Idaho Springs to Boulder. It went up Virginia Canyon on the aptly named Oh My God Road, and then through Central City to Blackhawk. This section from Idaho Springs to Blackhawk was changed to the present, easier alignment in 1938, with the southern terminus no longer in Idaho Springs but instead at a road junction with US 6 several miles to the east in Clear Creek Canyon.

In 1939 Route 119 was extended from Boulder to Longmont, zig-zagging along existing county roads. By 1960 it had been completely paved and straightened on a diagonal route (hence the name "Diagonal Highway") alongside the railroad tracks between Boulder and Longmont. When Interstate 25 was built east of Longmont by 1963, Route 119 was extended by overlapping US 287 to Third Avenue, where it replaced the previous Route 254 to connect to I-25. In 1998, the somewhat confusing triangle intersection with US 6 was simplified to a T intersection controlled by a traffic signal, and the west side of the triangle was abandoned and its tunnel boarded up. In 2003, a new expressway alignment for Route 119 was opened in Longmont, connecting directly from its intersection with US 287 to its terminus at I-25, bypassing the previous congested overlap with US 287.

CDOT plans various safety upgrades for the section between US 6 and Blackhawk. A diamond interchange is planned for the very busy intersection with SH 52 to ease the congestion at the large IBM facility.