SM U-18 was one of 329 submarines serving in the Imperial German Navy in World War I. U-18 engaged in the commerce warfare in the First Battle of the Atlantic.

Beginning in October 1914, she was commanded by Kaptlt. von Hennig.

On her third mission, on 23 November U-18 penetrated the fleet anchorage of Scapa Flow via Hoxa Sound, following a steamer through the boom and entering the anchorage with little difficulty. However, the fleet was absent, being dispersed in anchorages on the west coast of Scotland and Ireland. As U-18 was making her way back out through Hoxa Sound to the open sea, her periscope was spotted by a guard boat. The trawler Dorothy Gray altered course and rammed the periscope, rendering it unserviceable. U-18 then suffered a failure of her diving plane motor and the boat became unable to maintain her depth, at one point even impacting the seabed. She was rammed a second time by destroyer HMS Garry and eventually, her captain was forced to surface and scuttle his command just outside the Hoxa Gate; all crew members, except one, were picked up by British boats. One crew member perished, while the remaining 22 were interned as prisoners of war.

The wreck lies 75 m (246 ft) below the surface just outside the Hoxa Gate, at 58°41′N 02°55′W / 58.683°N 2.917°W / 58.683; -2.917.

• Gröner, Erich; Jung, Dieter; Maass, Martin (1991). U-boats and Mine Warfare Vessels. German Warships 1815–1945. Vol. 2. Translated by Thomas, Keith; Magowan, Rachel. London: Conway Maritime Press. ISBN 0-85177-593-4.
• Spindler, Arno (1966) [1932]. Der Handelskrieg mit U-Booten. 5 Vols. Berlin: Mittler & Sohn. Vols. 4+5, dealing with 1917+18, are very hard to find: Guildhall Library, London, has them all, also Vol. 1-3 in an English translation: The submarine war against commerce.
• Beesly, Patrick (1982). Room 40: British Naval Intelligence 1914-1918. London: H Hamilton. ISBN 978-0241108642.
• Halpern, Paul G. (1918). A Naval History of World War I. New York: Routledge. ISBN 978-1857284980.
• Roessler, Eberhard (1997). Die Unterseeboote der Kaiserlichen Marine. Bonn: Bernard & Graefe. ISBN 978-3763759637.
• Schroeder, Joachim (2002). Die U-Boote des Kaisers. Bonn: Bernard & Graefe. ISBN 978-3763762354.
• Koerver, Hans Joachim (2008). Room 40: German Naval Warfare 1914-1918. Vol I., The Fleet in Action. Steinbach: LIS Reinisch. ISBN 978-3-902433-76-3.
• Koerver, Hans Joachim (2009). Room 40: German Naval Warfare 1914-1918. Vol II., The Fleet in Being. Steinbach: LIS Reinisch. ISBN 978-3-902433-77-0.
• Kemp, Paul (1997). U-boats destroyed, German submarine losses in the World Wars. London: Arms & Armour Press. p. 9. ISBN 1-85409-321-5.

• Helgason, Guðmundur. "WWI U-boats: U 18". German and Austrian U-boats of World War I - Kaiserliche Marine - Uboat.net.
• Room 40: Archived 2018-10-15 at the Wayback Machine original documents, photos and maps about World War I German submarine warfare and British Room 40 Intelligence from The National Archives, Kew, Richmond, UK.