In Washington, D.C., Constitution Avenue is a major east-west street running just north of the United States Capitol in the city's Northwest and Northeast quadrants. The avenue carries heavy commuter traffic on weekdays and heavy tourist traffic on the weekends; it is also an important parade route.

In Washington's Cartesian-coordinate-based street system, Constitution Avenue was originally known as North B Street. If it had stayed in the city's lettered street system, it would today be known as B Street, N.W. and N.E. However, Independence Avenue was originally known as South B Street, and having two major streets running in parallel, both called B Street, would presumably be too confusing for many people. The street on the other side of the Anacostia River corresponding to Constitution Avenue is called Blaine Street, N.E.

Sections of Constitution Avenue are designated U.S. Highway 1, U.S. Highway 50, or both. Specifically, US 50 runs along the road from its west end to Sixth Street, N.W. (eastbound) and Ninth Street, N.W. (westbound). US 1 northbound uses the eastbound lanes of Constitution Avenue from 14th Street, N.W. to Sixth Street, N.W.; southbound US 1 used to run west from Ninth Street N.W. to 15th Street, N.W. but now continues straight through the Ninth Street Tunnel to I-395.

The western terminus of Constitution Avenue is the Theodore Roosevelt Bridge; thus, Constitution Avenue connects the city's ceremonial core with Interstate 66. The eastern terminus is at 21st Street, N.E., just short of Robert F. Kennedy Memorial Stadium; however, through traffic is diverted via North Carolina Avenue and C Street to the East Capitol Street Bridge.

In Northwest, Constitution Avenue separates the National Mall from the Federal Triangle. Thus, the avenue is lined on its north side by the headquarters of several federal agencies and on the south side by several Smithsonian museums and the National Gallery of Art. Other locations of interest include the Embassy of Canada. In Northeast, the avenue passes through the Capitol Hill and Lincoln Park neighborhoods.