Unterseeboot 556 was a Type VIIC U-boat of the Kriegsmarine. Her keel was laid down on 2 January 1940 by Blohm + Voss of Hamburg. She was commissioned on 6 February 1941, with Kapitänleutnant Herbert Wohlfarth in command. Wohlfarth commanded her for her entire career.

U-556 and the battleship Bismarck were neighbors in the ways at Blohm + Voss as they finished construction at about the same time. (Bismarck was commissioned on 24 August 1940.) In January 1941, as U-556's commissioning ceremonies approached, Wohlfarth wanted a band for the celebration, but could not afford to hire one. Kapitän Ernst Lindemann, commanding officer of Bismarck, loaned him his ship's band.

As thanks, Wohlfarth drew up a humorous Patenschaftsurkunde (Certificate of Sponsorship) promising that U-556 would protect Bismarck. A drawing shows Wohlfarth as the knight Parzival (his nickname) on the deck of U-556 simultaneously shooting down planes with a pistol and reaching underwater to stop a torpedo with his thumb. A second drawing then shows the submarine towing the battleship to safety.

The text accompanying the drawing reads:

Wir U556 (500 to) erklären hiermit vor Neptun, dem Herrscher über Ozeane, Meere, Seen, Flüsse, Büche, Teiche und Rinnsale daß wir unserem grossen Bruder, dem Schlachtschiff Bismarck (42.000 to) in jeder Lage, zu Wasser, unter Wasser, zu Lande wie in der Luft beistehen wollen.
Hamburg, den 28 Januar 1941
Kommandant u
Besatzung U556
We, U-556 (500 tons), hereby declare before Neptune, Lord over oceans, seas, lakes, rivers, brooks, ponds, and rivulets, that we will provide any desired assistance to our Big Brother, the battleship Bismarck (42,000 tons), at any place on the water, under water, on land, or in the air.
Hamburg, 28 January 1941
Commander &
Crew U556
U-556 conducted two patrols, sinking six ships totalling 29,552 tons and damaging one other grossing 4,986 tons.

On 26 May 1941, just four months after Wohlfahrt had sworn to defend Bismarck, U-556 was returning from patrol, low on fuel and entirely out of torpedoes, when she was ordered to reconnoiter in the area of Bismarck's most recently reported position.

Around 19:50, Wohlfahrt saw the battleship HMS Renown and the aircraft carrier HMS Ark Royal coming out of the mist at high speed. He recorded in his log, "Enemy bows on, 10 degrees to starboard, without destroyers, without zigzagging," but without torpedoes, could only submerge and avoid them. Wohlfahrt saw activity on Ark Royal's flight deck, which transpired to be the launching of the second, fatal attack on Bismarck.

At 20:39, Wohlfahrt surfaced and transmitted, "Enemy in view, a battleship, an aircraft carrier, course 115, enemy is proceeding at high speed. Position 48° 20' N, 16° 20' W." The course of Renown and Ark Royal toward Bismarck coincided almost exactly with his own; he proceeded on the surface at full speed behind them.

Wohlfahrt's War Diary contains these entries for 27 May 1941:

00:00, (wind) northwest 5, seaway 5, rain squalls, moderate visibility, very dark night. Surfaced. What can I do for Bismarck? I can see star shells being fired and flashes from Bismarck's guns. It is a terrible feeling to be near and not to be able to do anything. All I can do is reconnoiter and lead in boats that have torpedoes. I am keeping contact at the limit of visibility, reporting the position, and sending directional signals to call up the other boats.
03:52: I am moving around on the east side to the south, in order to be in the direction of the activity. I soon reach the limit of what I can do in view of my fuel supply. Otherwise I won't get home.
04:00: The seas are rising ever higher. Bismarck still fighting. Reported weather for the Luftwaffe.
Around 06:30 Wohlfahrt sighted U-74 and transferred the mission of maintaining contact with Bismarck to Kapitänleutnant Eitel-Friedrich Kentrat. He gave Kentrat Bismarck's position based on his observations of the star shells fired during the night, adding: "I have not seen her directly. You assume contact. I have no more fuel."

Wohlfahrt then submerged and did not surface again until noon, a time at which radio signals were routinely repeated. That was when he heard for the first time the order radioed to him between 07:00 and 08:00 to pick up Bismarck's War Diary. He replied to the Befehlshaber der Unterseeboote (Commander-in-Chief for Submarines, Karl Dönitz) asking that this mission be transferred to Kentrat.

Kentrat received the radio order, "U-boat Kentrat pick up Bismarck War Diary," but was unable to locate Bismarck. The battleship had sunk before Wohlfahrt had received the first message at noon.

U-556 did not suffer any casualties to her crew during her career until 27 June 1941, when depth charges from the British corvettes Nasturtium, Celandine, and Gladiolus sank her in the North Atlantic south-west of Iceland. Five died, 41 survived.