The foundation date is unknown. Rendsburg was first mentioned in 1199. It became a part of Holstein in the 13th century, but was passed to Schleswig in 1460. Many times the town changed from the Danish to the German domain and vice versa. In the German-Danish War in 1864 Rendsburg was finally seized by Kingdom of Prussia and Austria. After 1866 annexed by Kingdom of Prussia, since that time remained German.
The importance rose in 1895, when the Kiel Canal was finished. Although situated inland, Rendsburg received a seaport and a dockyard. The most prominent building of the town, a railway bridge made of steel, 2500 m in length and 41 m in height, was constructed in 1913. A ferry crossing the Kiel Canal below the bridge is anchored to the bridge by steel cables, a unique type of construction in Europe.