U.S. Route 240 is a defunct designation for a short, but once very important, segment of highway between Frederick, Maryland, and Washington, D.C.

US 240 began at the intersection of Pennsylvania Avenue and 14th Street, NW, near the White House in Washington, D.C.[citation needed] US 240 headed west on Pennsylvania Avenue to 15th Street, then headed west from that intersection on Treasury Place for one block (entering an area now closed to the public and known as President's Park).

By 1959, the starting point and routing of US 240 south of Massachusetts Avenue and 22nd Street (one block east of Sheridan Circle) had changed.

North of Bethesda, US 240 met the western end of MD 702 (Jones Bridge Road) just south of the National Naval Medical Center.

The highway began to parallel the Baltimore and Ohio Railroad (now CSX's Metropolitan Subdivision) as it approached the town of Rockville.

US 240 entered the county seat at Dodge Street and had an acute intersection with MD 28 (Viers Mill Road).

US 240 crossed Little Seneca Creek and had a pair of staggered intersections with MD 121 (Clarksburg Road); the latter highway followed what are now Spire Street and Redgrave Place north and south, respectively.

Just north of Hyattstown, US 240 crossed the Montgomery–Frederick county line and met the southern end of MD 75 (Green Valley Road).

US 240 crossed Bennett Creek as it approached Urbana, through which the highway ran concurrently with MD 80 (Fingerboard Road).

Highways crossed Carroll Creek before US 240 ended at US 40 (Patrick Street) in the center of downtown Frederick.