Unterseeboot 110 (U-110) has been the designation of two submarines of the German Navy.

The first U-110 — World War I

During World War I, U-110 was launched on July 28, 1917, and commissioned into the Kaiserliche Marine on September 25, 1917.

The second U-110 — World War II

The second U-110 was a Type IXB submarine of the Kriegsmarine. Her keel was laid down February 1, 1940 by AG Weser, of Bremen, Germany. She was commissioned November 21, 1940 with Kapitänleutnant Fritz-Julius Lemp in command. Lemp commanded her for her entire career.

The second U-110 conducted two patrols, sinking three ships totalling 10,149 tons and damaging two others totalling 8,675 tons. On March 23, 1941, her 105mm deck gun exploded during firing, wounding three men. On May 9, 1941, she was captured.

U-110 was attacking a convoy along with U-201 in the North Atlantic south of Iceland when Lemp left his periscope up too long. The escort HMS Aubretia spotted it and began dropping depth charges. U-110 survived the first attacks but when HMS Bulldog and HMS Broadway joined the attack, U-110 was forced to surface. Bulldog set course to ram. Lemp saw her charge and ordered "Abandon Ship," presumably assuming that since the boat was going to be rammed and sunk, its secrets — such as codebooks and Enigma settings lists — were safe.

At the same time, however, Bulldog 's commander Captain Joe Baker-Cresswell realized that a capture might be possible and changed course, avoiding the collision. Lemp, in turn, saw that the boat was not sinking and attempted to swim back to it to destroy the secret material. He was never seen again. He may have been shot in the water by a British sailor (as testified by a German eyewitness), but his fate is unknown. Including Lemp, 15 men were killed in the action and 32 captured.

Bulldog's crew stripped U-110 of everything portable on the spot, including her secret documents and Enigma cipher machine. U-110 was taken in tow back toward England, but to ensure that the Germans would not learn that she had been captured and change the cipher system, the Admiralty send orders that the boat be scuttled at sea.

U110's capture was later given the code word Operation Primrose and was one of the biggest secrets of the war. It remained secret for thirty years. President Roosevelt was only told by Churchill in January 1942.

The movie U-571 was partially inspired by the capture of U-110.

See also: List of U-boats