HMS Dreadnought was a Royal Navy battleship, the design of which revolutionised naval power. The ship's entry into service in 1906 represented such an advance in naval technology that her name came to be associated with an entire generation of battleships, the Dreadnoughts, as well as the class of ships named after her. Likewise, the generation of ships she made obsolete became known as pre-dreadnoughts. Admiral Sir John "Jacky" Fisher, First Sea Lord of the Board of Admiralty, is credited as the father of Dreadnought. Shortly after he assumed office in 1904, he ordered design studies for a battleship armed solely with 12 in (305 mm) guns and a speed of 21 knots (39 km/h; 24 mph). He convened a Committee on Designs to evaluate the alternative designs and to assist in the detailed design work.

Dreadnought was the first battleship of her era to have a uniform main battery, rather than having a few large guns complemented by a heavy secondary armament of smaller guns. She was also the first capital ship to be powered by steam turbines, making her the fastest battleship in the world at the time of her completion. Her launch helped spark a naval arms race as navies around the world, particularly the Imperial German Navy, rushed to match it in the build-up to the First World War.

Although designed to engage enemy battleships, her only significant action was the ramming and sinking of German submarine SM U-29; thus she became the only battleship confirmed to have sunk a submarine. Dreadnought did not participate in the Battle of Jutland in 1916 as she was being refitted, nor did she participate in any of the other naval battles in World War I. In July 1916 she was relegated to coastal-defence duties in the English Channel, before rejoining the Grand Fleet in 1918. The ship was reduced to reserve in 1919 and sold for scrap two years later.

Gunnery developments in the late 1890s and the early 1900s, led in the United Kingdom by Percy Scott and in the United States by William Sims, were already pushing expected battle ranges out to an unprecedented 6,000 yd (5,500 m), a distance great enough to force gunners to wait for the shells to arrive before applying corrections for the next salvo. A related problem was that the shell splashes from the more numerous smaller weapons tended to obscure the splashes from the bigger guns. Either the smaller-calibre guns would have to hold their fire to wait for the slower-firing heavies, losing the advantage of their faster rate of fire, or it would be uncertain whether a splash was due to a heavy or a light gun, making ranging and aiming unreliable. Another problem was that longer-range torpedoes were expected soon to be in service and these would discourage ships from closing to ranges where the smaller guns' faster rate of fire would become preeminent. Keeping the range open generally negated the threat from torpedoes and further reinforced the need for heavy guns of a uniform calibre.

In 1903, the Italian naval architect Vittorio Cuniberti first wrote about the concept of an all-big-gun battleship. When the Italian Navy did not pursue his ideas, Cuniberti wrote an article in Jane's Fighting Ships advocating his concept. He proposed an "ideal" future British battleship of 17,000 long tons (17,000 t), with a main battery of a dozen 12-inch guns in eight turrets, 12 inches of belt armour, and a speed of 24 knots (44 km/h; 28 mph).

The Royal Navy, the Imperial Japanese Navy and the United States Navy all recognised these issues before 1905. The Royal Navy modified the design of the Lord Nelson-class battleship to include a secondary armament of 9.2 in (234 mm) guns that could fight at longer ranges than the 6 in (152 mm) guns on older ships, but a proposal to arm them solely with 12-inch guns was rejected. The Japanese battleship Satsuma was laid down as an all-big-gun battleship, five months before Dreadnought, but gun shortages allowed her to be equipped with only four of the twelve 12-inch guns that had been planned. The Americans began design work on an all-big-gun battleship around the same time in 1904, but progress was leisurely and the two South Carolina-class battleships were not ordered until March 1906, five months after Dreadnought was laid down, and the month after she was launched.

The invention by Charles Algernon Parsons of the steam turbine in 1884 led to a significant increase in the speed of ships with his dramatic unauthorised demonstration of his yacht Turbinia with her speed of up to 34 knots (63 km/h; 39 mph) at Queen Victoria's Diamond Jubilee at Spithead in 1897. After further trials of two turbine-powered destroyers, Viper and Cobra, coupled with the positive experiences of several small passenger ships with turbines, Dreadnought was ordered with turbines.

The Battle of the Yellow Sea and Battle of Tsushima were analysed by Fisher's Committee, with Captain William Pakenham's statement that "12-inch gunfire" by both sides demonstrated hitting power and accuracy, whilst 10-inch shells passed unnoticed. Admiral Fisher wanted his board to confirm, refine and implement his ideas of a warship that had both the speed of 21 knots and 12-inch guns, pointing out that at the Battle of Tsushima, the Japanese ships commanded by Admiral Togo had been able to "cross the T" of the Russian ships due to speed. The long-range (14,000-yard (13,000 m)) engagement during the Battle of the Yellow Sea, in particular, although never experienced by any navy prior to the battle, seemed to confirm what the Royal Navy already believed.