Seydlitz was a heavy cruiser of Nazi Germany's Kriegsmarine, fourth in the Admiral Hipper class, but was never completed.

The ship was laid down in December 1936 and launched in January 1939, but the outbreak of World War II slowed her construction and fitting-out work was finally stopped in the summer of 1940 when she was approximately 95 percent complete.

The unfinished ship remained pier-side in the shipyard until March 1942, when the Kriegsmarine decided to pursue aircraft carriers over surface combatants.

The Admiral Hipper class of heavy cruisers was ordered in the context of German naval rearmament after the Nazi Party came to power in 1933 and repudiated the disarmament clauses of the Treaty of Versailles.

The ship also carried a pair of triple 53.3 cm (21 in) torpedo launchers abreast of the rear superstructure.

The Kriegsmarine decided, however, to complete the ship identically to Admiral Hipper on 14 November 1936.

After the outbreak of World War II in September 1939, work slowed considerably as priorities shifted to vessels that could be completed faster, like destroyers, U-boats, and smaller craft.

[11] Work on the purpose-built carrier Graf Zeppelin, which had been halted in April 1940, was resumed in March 1942.

The navy envisioned operating the carriers from northern Norway to interdict the supply convoys to the Soviet Union.

[13] At the same time as construction of Graf Zeppelin resumed, conversion work began on Seydlitz.

[14] The majority of the superstructure was cut away, with the exception of the funnel, to prepare for the installation of a flight deck and an aircraft hangar.

[16] As British and American air raids increased in intensity through 1943, the Germans decided to move the ship to Königsberg in an attempt to place her out of the reach of Allied bombers.

[16] On 29 January 1945, the ship was scuttled before the advancing Soviet Red Army could seize her.

[20] Her armament was reduced to a medium-caliber anti-aircraft battery of ten 10.5 cm L/65 guns in twin mounts, two forward of the conning tower and three aft.

[18] The mounts were the Dopp LC/31 type, originally designed for earlier 8.8 cm (3.5 in) SK C/31 guns.