SMS Seydlitz was a battlecruiser of the German Kaiserliche Marine (Imperial Navy), built in Hamburg. She was ordered in 1910 and commissioned in May 1913, the fourth battlecruiser built for the High Seas Fleet. Named after Friedrich Wilhelm von Seydlitz, Seydlitz represented the culmination of the first generation of German battlecruisers, which had started with the Von der Tann in 1906 and continued with the pair of Moltke-class battlecruisers ordered in 1907 and 1908. Seydlitz featured several incremental improvements over the preceding designs, including a redesigned propulsion system and an improved armor layout. The ship was also significantly larger than her predecessors—at 24,988 metric tons (24,593 long tons; 27,545 short tons), she was approximately 3,000 metric tons heavier than the Moltke-class ships.

Seydlitz participated in many of the large fleet actions during World War I, including the battles of Dogger Bank and Jutland in the North Sea. The ship suffered severe damage during both engagements; during the Battle of Dogger Bank, a 13.5 in (34.3 cm) shell from the British battlecruiser Lion struck Seydlitz's rearmost turret and nearly caused a magazine explosion that could have destroyed the ship. At the Battle of Jutland she was hit twenty-one times by large-caliber shells, one of which penetrated the working chamber of the aft superfiring turret. Although the resulting fire destroyed the turret, the safety measures imposed after the battle of Dogger Bank prevented a catastrophe. The ship was also hit by a torpedo during the battle, causing her to take in over 5,300 metric tons of water and her freeboard was reduced to 2.5 m. She had to be lightened significantly to permit her crossing of the Jade Bar. The ship inflicted severe damage on her British opponents as well; early in the battle, salvos from both Seydlitz and the battlecruiser Derfflinger destroyed the battlecruiser Queen Mary in seconds.

Seydlitz saw limited action in the Baltic Sea, when she provided screening for the German flotilla that at Battle of the Gulf of Riga attempted to clear the gulf in 1915. As with the rest of the German battlecruisers that survived the war, the ship was interned in Scapa Flow in 1918. The ship, along with the rest of the High Seas Fleet, was scuttled in June 1919, to prevent her seizure by the British Royal Navy. She was raised on 2 November 1928 and scrapped by 1930 in Rosyth.

In early 1909, discussions began among the leadership of the German Kaiserliche Marine (Imperial Navy) over the next battlecruiser to be built, which would follow the two Moltke-class battlecruisers that were then still under construction. By that time, clear design philosophies had emerged in the two navies that were building battlecruisers: the British Royal Navy preferred heavily armed, but lightly armored, ships, while the Germans favored heavily armored, but somewhat more lightly armed vessels. Korvettenkapitän (Corvette Captain) Waldemar Vollerthun, then the head of the navy's General Department, wrote a memorandum in March 1909 that succinctly explained the difference as "the 'English Battleship-Cruisers' stand against our 'Cruiser-Battleships'." Vollerthun argued in the memorandum that the two Moltke-class ships would need to complete their sea trials before design work on the next ship—provisionally titled "J"—could begin. Furthermore, he stated that developments in dreadnought battleship design indicated that the caliber of the main battery should be increased from 28 to 30.5 cm (11 to 12 in) (though he conceded that the former gun was still sufficient against foreign capital ships). He also recommended lowering the armor deck to save weight.

The navy's Construction Department issued its own memo on the same day as Vollerthun's document. They disagreed with lowering the armor deck, but suggested the adoption of triple gun turrets for the main battery (instead of the twin turrets used in previous vessels). The Weapons Department rejected the latter proposal. An initial conference was held on 4 April to discuss parameters for the new ship. The Budget Section informed the participants that no increase in cost could be permitted over Moltke. The project saw no further work until 13 August, when the Construction Department reminded the other departments that deadlines were approaching if the navy wanted work to begin in 1910. On 28 August, the departments were informed that an additional 750,000 marks would be made available by a public fundraising effort; this money could be used to make improvements over the Moltke design. Admiral Alfred von Tirpitz, the head of the Reichsmarineamt (Imperial Navy Office) requested a preliminary design be made available by 27 September, so he could present it to Kaiser Wilhelm II for consideration. Tirpitz's criteria were as follows: armament would remain the same as Moltke (ten 28 cm guns); displacement and length should only moderately increase from the earlier ship.

The naval engineer Hans Bürkner submitted a pair of proposals on 23 September. The first, "I", was an improvement over Moltke, but with increased armor protection and a slightly longer hull to offset the greater displacement of about 700 metric tons (690 long tons; 770 short tons) and maintain the same maximum speed. The second, "II", had the same armor improvements as "I" but with the armor deck lowered as Vollerthun had suggested to keep displacement within limits, though it had increased by 1,000 t (980 long tons; 1,100 short tons) compared to Moltke. A third option, "III", reverted to the four-turret arrangement of Von der Tann but increased the caliber to 30.5 cm; three subvariants were included with slightly different armor arrangements. Two days later, the Construction Department provided the proposals to Tirpitz, who postponed a final decision; he was still unsure of whether ordering a third member of the Moltke class was preferable, since it could be built faster than a new design. In the meantime, the Construction Department produced a revised design, "IIb" on 8 October; a slight increase in displacement bought improved internal subdivision of the propulsion machinery spaces.

After a conference with the Kaiser at Rominten, Tirpitz agreed that "IIb" should be adopted, but offered an increase in displacement from 24,000 to 24,700 t (23,600 to 24,300 long tons; 26,500 to 27,200 short tons) for additional improvements, primarily an increase in armor for the propulsion machinery spaces. Though the debate appeared to be settled, the Construction and General Departments then submitted a fourth design, "IVe", which significantly revised the layout of the main battery; one of the wing turrets was moved forward to a superfiring position over the fore turret, and the other wing mount was moved to the centerline. This arrangement provided increased firepower forward and maintained the same broadside. Bürkner wrote a memorandum on 13 December comparing "IIc" and "IVe"; in addition to the improvements in offensive power, it was structurally superior. The General Department issued its own evaluation on 20 December, which concluded that "IVe" was preferable, even if it had somewhat worse internal subdivision. The following day, Tirpitz convened a meeting to discuss the designs. The need to proceed quickly to take advantage of a favorable negotiating position with Blohm & Voss led those present to decide in favor of "IIc". On 7 January 1910, settled on a slightly improved version, "IIe", which raised the forecastle and made a few minor increases to the armor layout. The Kaiser approved the proposal on 27 January.

Seydlitz was 200 meters (656 ft 2 in) long at the waterline, and 200.6 m (658 ft 2 in) overall. The ship had a beam of 28.5 m (93 ft 6 in), which was increased to 28.8 m (94 ft 6 in) with anti-torpedo nets equipped. She had a draft of 9.29 m (30 ft 6 in) forward and 9.09 m (29 ft 10 in) aft. Seydlitz displaced 24,988 metric tons (24,593 long tons) as designed, which increased to 28,550 t (28,100 long tons) at full load. Seydlitz had a double bottom for 76 percent of the length of the hull. Her hull was divided into seventeen watertight compartments. The entire structure consisted of Siemens-Martin mild steel. All internal decks were fitted with linoleum, while the external decks were layered with teak planking.