Delmar is a town in Sussex County, Delaware, United States, on the Maryland border along the Transpeninsular Line. Its motto is "The Little Town Too Big for One State." The population was 1,597 at the 2010 census, an increase of 13.5% over the previous decade. It is part of the Salisbury, Maryland-Delaware Metropolitan Statistical Area and a suburb of Salisbury, MD. When added with Delmar, Maryland, the total population of the town was 4,600 at the 2010 Census.

The Delaware Railroad was the major railroad in the US state of Delaware, traversing almost the entire state north to south. It was planned in 1836 and built in the 1850s. It began in Porter and was extended south through Dover, Seaford and finally reached Delmar on the border of Maryland in 1859. Although operated independently, in 1857 it was leased by and under the financial control of the Philadelphia, Wilmington, and Baltimore Railroad. In 1891, it was extended north approximately 14 miles (23 km) with the purchase of existing track to New Castle and Wilmington. With this additional track, the total length was 95.2 miles (153.2 km).