U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. The term is an anglicised version of the German word U-Boot, a shortening of Unterseeboot (under-sea boat), though the German term refers to any submarine. Austro-Hungarian Navy submarines were also known as U-boats.

U-boats are most known for their unrestricted submarine warfare in both world wars, trying to disrupt merchant traffic towards the UK and force the UK out of the war. In World War I, Germany intermittently waged unrestricted submarine warfare against the UK: a first campaign in 1915 was abandoned after strong protests from the US but in 1917 the Germans, facing deadlock on the continent, saw no other option than to resume the campaign in February 1917. The renewed campaign failed to achieve its goal mainly because of the introduction of convoys. Instead the campaign ensured final defeat as the campaign was a contributing factor to the entry of the US in the First World War.

 In World War II, Karl Dönitz supreme commander of the Kriegsmarine's U-boat arm (Befehlshaber der Unterseeboote), was convinced the UK and its convoys could be defeated by new tactics, and tried to focus on convoy battles. Ultimately the U-boats were defeated in May 1943.