List of Austro-Hungarian U-boats

The Austro-Hungarian U-boat fleet during the First World War mainly consisted of German manufactured units transported by rail from Germany's northern shipyards to the Austrian ports on the Adriatic Sea. They served throughout the war against Italian, French and British shipping in the Mediterranean Sea with some success, losing eight of the twenty eight boats in service in return. Following the end of the war in 1918, all Austrian submarines were surrendered to the Entente powers, who disposed of them individually. As both Austria and Hungary became landlocked in the aftermath of the war, no Austrian or Hungarian submarines (or any other naval vessels) have been commissioned since.

In some sources Austro-Hungarian U-boats are referenced with Roman numerals as a way to distinguish them from German U-boats with similar numbers, but the Austro-Hungarian Navy itself used Arabic numerals. There are gaps in the numbering for several reasons. One series of Austro-Hungarian U-boats under construction in Germany was sold and commissioned into the Imperial German Navy. In other cases, U-boats commissioned into the Imperial German Navy were temporarily assigned Austro-Hungarian numbers when they operated in the Mediterranean. One final reason, in the case of the unassigned U-13, was superstition.

Other
Submarines on which construction was begun but which were not completed or commissioned during World War I are included in this table.

German U-boats operating under the Austro-Hungarian flag
After Italy had entered World War I by declaring war on Austria-Hungary on 23 May 1915, Germany felt treaty-bound to support the Austrians in attacks against Italian ships, even though Germany and Italy were not officially at war. As a result, German U-boats operating in Mediterranean were assigned Austro-Hungarian numbers and flags. In some cases the same Austro-Hungarian numbers were assigned to different German U-boats. After 28 August 1916, when Germany and Italy were officially at war, the practice continued, primarily to avoid charges of flag misuse. The practice was largely ended by 1 October 1916 except for a few large U-boats that continued using Austro-Hungarian numbers.