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. Seydlitz was among the vessels chosen for conversion into auxiliary aircraft carriers.

Renamed Weser, the ship was to have had a complement of ten Bf 109 fighters and ten Ju 87 divebombers. Work was not completed, however, and the incomplete vessel was towed to Königsberg where she was eventually scuttled. The ship was seized by the advancing Soviet Army and was briefly considered for cannibalization for spare parts to complete her sistership Lützow for the Soviet Navy. This plan was also abandoned, and the ship was broken up for scrap.