SH 121's southern terminus is at Waterton Road near Littleton, and the northern terminus is at U.S. Route 287 (US 287) in Broomfield.

The route, also known as Wadsworth Boulevard, Wadsworth Bypass, and Wadsworth Parkway, starts at a junction with Waterton Road in unincorporated Jefferson County (near Littleton and at the entrance to the main plant of Lockheed Martin Space Systems) and ends at the junction of U.S. Route 36, U.S. Route 287, and State Highway 128 in Broomfield at a trumpet interchange.

State Highway 121 passes through portions of southwest Denver County.

In northwest Arvada, Wadsworth Boulevard passes over Hackberry Hill at the site where a landmark hackberry tree, for which the hill was named, stood before it was cut down to make room for the highway in 1937.

[2] Media related to Colorado State Highway 121 at Wikimedia Commons