U-boat is the anglicized version of the German word U-Boot [ˈuːboːt] ( listen), itself an abbreviation of Unterseeboot ("undersea boat", or submarine), and refers to military submarines operated by Germany, particularly in World War I and World War II. Although in theory U-boats could have been useful fleet weapons against enemy naval warships, in practice they were most effectively used in an economic warfare role (commerce raiding), enforcing a naval blockade against enemy shipping. The primary targets of the U-boat campaigns in both wars were the merchant convoys bringing supplies from Canada, the British Empire and the United States to the islands of Great Britain. Austrian submarines of World War I (that is, before the country lost its coastline after the war)were also known as U-boats.

The distinction between U-boat and submarine is common in several languages, including English (where U-boat refers exclusively to the German vessels of the World Wars) but is unknown in German, in which the term U-Boot refers to any submarine.