Maryland Route 75 (MD 75) is a state highway in the U.S. state of Maryland. Known for most of its length as Green Valley Road, the state highway runs from MD 355 near Hyattstown north to MD 31 in New Windsor. MD 75 serves as the main north–south highway of eastern Frederick County, where it connects Hyattstown with New Market, Libertytown, Johnsville, and the Carroll County town of Union Bridge. The first sections of MD 75 were constructed around New Windsor and between Green Valley and New London in the early 1910s. The latter highway was extended south to Green Valley in the late 1910s. In the early 1920s, MD 75 was constructed between New Windsor and Union Bridge and from Libertytown to Johnsville. The remainder of the state highway was constructed in the late 1920s and early 1930s. MD 75 was relocated between Union Bridge and New Windsor in the early 1960s, around its interchange with Interstate 70 (I-70) and U.S. Route 40 (US 40) in New Market in the early 1970s, and at its northern terminus in New Windsor in 2007.