Haderslev (hăˈħərslĕv) [key], Ger. Hadersleben, city (1992 pop. 20,329), Sønderjylland co., S Denmark, a seaport on the Haderslev Fjord, an inlet of the Lille Bælt. It is a commercial and industrial center and rail junction. Haderslev was held by Prussia from 1864 to 1920. Of note is the Church of St. Mary (13th cent.).