Concord is an unincorporated community in Sussex County, Delaware, United States. Concord is located along Delaware Route 20 east of Seaford. Concord, Sussex County, Delaware should not be confused with Concord, New Castle County, Delaware; the latter is a populated place located in Wilmington, bordering the northern edge of the city along Delaware Route 202. The Pine Grove Furnace Site was listed on the National Register of Historic Places in 1978.