Maryland Route 121, also known as Clarksburg Road, is a Maryland state highway that goes from Boyds to Clarksburg in Montgomery County, through Black Hill Regional Park.

Maryland Route 121 begins at a wide three-way intersection with MD 117 near Boyds. The south and west legs of the intersection consist of MD 117 (Barnesville Road); the north leg is MD 121, following Clarksburg Road, which curves to the northeast on a two-lane, shouldered alignment. It very quickly encounters a bridge over Little Seneca Lake. The shoulders quickly end on the opposite shore and the road curves slightly more sharply as it passes through Burdette's cluster of homes. The route then enters the open countryside, weaving through farms and small patches of woodland before it passes through its interchange with Interstate 270. The MD 121 designation ends at the very last ramp of I-270, as does the road's name of Clarksburg Road. The roadway continues, however, as Stringtown Road, continuing for quite a distance north and ending at Kings Valley Road. A separated section of MD 121 officially designated MD 121A but signed as MD 121 begins at a dead end near of I-270 and heads northeast along Clarksburg Road. It intersects Gateway Center Drive, which connects to Stringtown Road, before coming to an end at MD 355.

The entire route is in Montgomery County.

The entire route is in Clarksburg, Montgomery County.