Rendsburg's foundation date is unknown though some form of fortifications was established by Bjørn Svendsen 1099–1100.

Although the Eider is navigable for small craft from its mouth on the North Sea to Rendsburg, the town's importance rose in 1895, when the Kiel Canal was finished.

The much larger ships that could navigate the Kiel Canal meant that, although situated inland, Rendsburg became a seaport and a dockyard.

The most prominent structure in town, the Rendsburg High Bridge, a railway bridge made of steel, 2,500 metres (8,200 ft) in length and 41 metres (135 ft) in height, was constructed in 1913 to take the Neumünster–Flensburg railway over the Kiel Canal from the relatively flat land on either side.

The German Army's Air Defence School and the Bundesamt für Strahlenschutz are both located in Rendsburg.