The Arabic geographer Muhammad al-Idrisi mentions in the mid-12th century a landmarkTu(r)ndira, which may be Tønder or possibly the nearby town of Mogeltonder. The Carta Marina of Olaus Magnus shows it as Tunner.
Tønder was granted Hanseatic port privileges in 1243, and is thus Denmark's oldest privileged market town. In 1532 the town was hit by severe flooding, with the water reaching 1.8 metres high in St Laurent's church, 5.3 metres above normal. Tonder's port lost direct access to the sea mainly due to the building of dykes west of the town in the 1550s, at the instigation of Duke Hans the Elder of Schleswig-Holstein-Haderslev (a son of Frederick I of Denmark). The centre of the town is dominated by houses from the late 17th and early 18th century, when the town experienced rapid growth as a result of its lace industry.
Prior to 1864, Tonder was situated in the Duchy of Schleswig, so its history is properly included in the contentious history of Schleswig-Holstein. In the 1920 Schleswig Plebiscite that incorporated Northern Schleswig as part of Denmark, 76.5% of Tønder's inhabitants voted for remaining part of Germany and 23.5% voted for the cession to Denmark.
During World War I, a base for Zeppelin airships was operated at Tonder by the German navy. The former site now houses a museum, the Zeppelin and Garrison Museum Tonder. The base was attacked by the British on 19 July 1918, during what is known as the Tondern raid. Seven Sop with Camels from the aircraft carrier HMS Furious bombed the base and two of the three Airship hangars were hit, the Zeppelins L.54 and L.60inside one hanger were destroyed and a balloon inside the other was damaged. After this, Tondern was abandoned as an active airship base, and ordered to be used only as an emergency landing site. A wartime aircraft hangar survives, as do some of the ancillary buildings from the period, but only the foundations remain of the large airship hangars.
During the World