Flensburg, Or Flensborg, a seaport and market town of the Prussian province of Schleswig-Holstein, at the head of Flensburg fiord, an inlet of the Baltic, 20 m. N. N. W. of Schles-wig; pop. in 1871, 21,325. It is the most populous town and the chief commercial mart of the former duchy of Schleswig. It manufactures sugar, tobacco, paper, soap, and iron, has breweries and distilleries, and builds ships for the West India trade. Railways connect it with the principal towns of the province, and with Kolding in Jutland, and steamships with Stettin and other ports of the Baltic. The harbor is deep enough for large craft, but is difficult of entrance. About 200 vessels, many of which are employed in the Greenland whale fishery, are owned here. The number of entrances in 1869 was 2,211. Flensburg was a wealthy town as early as the 12th century, but it afterward suffered much from wars and conflagrations.