The Transpeninsular Line (38° 27' N) is a surveyed line that forms the north-south border between Delaware and Maryland. The border turns north from the midpoint of the line towards the Twelve-Mile Circle, which forms the remainder of the Delaware border.

In 1751, a line was surveyed straight across the Delmarva Peninsula beginning at what they called Cape Henlopen, near Lewes, Delaware, which was to be the southern boundary of Delaware. The surveyors, however, actually began at Fenwick Island, fifteen miles south. The line was accepted by Charles Mason and Jeremiah Dixon (of Mason-Dixon line fame) in 1763 when they were engaged to determine the borders between Maryland and the grants to William Penn (Pennsylvania and Delaware).

The line splits several communities on the Delmarva, separating them between the two states. The best-known of these towns are Delmar, Delaware, and Delmar, Maryland, which derive their name from the two states' names.

A Transpeninsular Line marker can be found near the Fenwick Island Lighthouse , at the northern boundary of Ocean City, Maryland.