Ada Kaleh (Romanian pronunciation: [ˈada kaˈle], Turkish: Adakale "Island Fortress", Hungarian: Újorsova or Ada Kaleh, Serbian: Адакале / Adakale) was a small island on the Danube populated mostly by Turks that was submerged during the construction of the Iron Gates hydroelectric plant in 1970. The island was about 3 km downstream from Orşova and measured 1.75 by 0.4–0.5 km.

The isle of Ada Kaleh is probably the most evocative victim of the Iron Gate dam's construction. A Turkish enclave, it had a mosque and numerous twisting alleys, and was known as a free port and a smuggler's nest. Many other ethnic groups lived here beside the Turks.

The Austrians built a Vauban-type fort there to defend it from the Ottoman Empire, and that fort would remain a bone of contention for the two empires. In 1699 the island came under Turkish control; but from 1716 to 1738 it was recaptured by the Austrians, and the Fortress of New Orsova was built by Nicolaus Doxat de Démoret, an Austrian colonel of Swiss origin. After a four-month siege in 1738 it was Turkish again, followed by the Austrians re-conquering it in 1789, but they had to return the island to the Turks with the Treaty of Sistova on August 4, 1791, which ended the Austro-Turkish War (1787–1791) (and, by extension, the Ottoman–Habsburg wars) between the Ottoman Empire and Habsburg Monarchy. Thereafter, the island lost its military importance.