U-73 was a German submarine, which was built in Gdansk (Danzig in German), Poland, in November 1915. She was one of the 329 submarines which joined the Imperial German Navy in World War I. She participated in the war in the context of the First Battle of the Atlantic. Her commander was Captain-Lieutenant (Gustav Siess). Until February 1916 she remained in Kiel, Germany, for testing and crew training. Then she sailed to the North Sea and joined the 1st Flotilla. Her first operational mission began on April 1, 1916, when she sailed from the bay of Heligoland, Germany, to the Mediterranean via the North Sea. While sailing, she attacked a steamship in the Atlantic and laid mines off Lisbon and Malta. On April 27, 1916, she dumped a minefield of 22 mines outside the Great Harbour of Valletta, Malta, where four ships sank: the HMS Russell battleship, the HMS Nasturtium minesweeper, the HMT Crownsin, which sank on May 4, 1916, with the loss of 11 men, as well as the HMY Aegusa yacht. Upon arrival at Kotor (gulf in Montenegro), in early May, U-73 joined the Pola-Kotor Fleet.

The minelaying operations of U-73 in the Mediterranean are not completely known. On October 7, 1916, she reportedly sailed from Pola in Croatia and the French held her responsible for the mine that was dumped at cape Maleas on October 12 as well as minefields in the gulf of Thessalonike and the Saronic Gulf, in which two Greek ships sank. It seems certain that U-73, which continued to be commanded by Siess, dumped the mine due to which the 48.158-ton HMHS Britannic hospital ship sank, kin ship to Titanic, the largest passenger ship, which currently lies on the seabed and the largest ship that sank during World War I, just an hour after U-73 laid the mine. It is also considered very probable that the same minefield is considered responsible for the sinking, on November 14, 1916, of the SS Burdigala French troopship, which sailed from Thessalonike to Toulon in order to receive troops. It is likely that the HMHS Braemar Castle hospital ship was also damaged by her mines.

U-73 suffered from constant problems with her hardware and other devices, just like all the ships of her series. At the end of October 1918, commanded by Captain-Lieutenant Fritz Saupe, she was scuttled at Pola in Croatia.