Maryland Route 646 (MD 646) is a state highway in the U.S. state of Maryland. Known as Prospect Road, the state highway runs 3.31 miles (5.33 km) from MD 543 near north to MD 136 at . MD 646 was constructed as a section of MD 543 in the early 1930s. That section of MD 543 and the road originally marked as MD 646 swapped numbers by 1946.