State Highway 21 is an expressway in El Paso County, in eastern Colorado Springs. The highway serves as an eastern bypass for the city of Colorado Springs. Officially, the routing is from I-25 at Northgate Blvd in the north to Drennan Rd in the south.

For most of its routing the highway is a four to six-lane, limited access highway. In the north, it begins as a semi-freeway with a 60 mph speed limit, and becomes a six lane expressway with a 55 mph speed limit in the city of Colorado Springs after Union Boulevard and remains so throughout the entire route. The northern section, from Research Parkway to Interquest Parkway/SH 83 is constructed to interstate highway standards with sufficient right-of-way to construct planned overpasses and on/off-ramps. CDOT has begun the project of constructing interchanges and bridges at Old Ranch Road, Union Boulevard, and Briargate Parkway, and when completed, will make the corridor from SH 83 to Research Parkway a non-stop freeway. An interchange with Research Parkway and with others will be constructed at a later date. A freeway to freeway interchange is planned for the SH 21/I-25/Northgate Blvd interchange, though the road currently ends at Interquest Parkway/SH 83. The whole routing from I-25 to Proby Parkway is planned as a controlled-access freeway. The southern portion (from Research Parkway) is a limited-access highway with interchanges constructed at Woodmen Road and Platte Ave/US 24. It ends at Mesa Ridge Parkway in Security-Widefield.

The route was established on October 1, 2007.When defined, its routing included the entire length of Powers Boulevard.

The entire route is in El Paso County.