Maryland Route 138 (MD 138) is a state highway in the U.S. state of Maryland. The state highway runs 9.24 miles (14.87 km) from MD 45 in Hereford east to MD 23 near Shawsville. MD 138 connects northern Baltimore County with northwestern Harford County via the community of Monkton on Gunpowder Falls. The first section of the state highway was built east of Monkton in the 1910s. The remainder of MD 138 was built west of Monkton in the mid-1920s and east of Monkton in the early to mid-1930s. The state highway through Monkton was maintained by Baltimore County from the late 1960s to the mid-1990s, during which the highway was relocated at Gunpowder Falls.