HMS Neptune was a dreadnought battleship built for the Royal Navy in the first decade of the 20th century, the sole ship of her class. She was the first British battleship to be built with superfiring guns. Shortly after her completion in 1911, she carried out trials of an experimental fire-control director and then became the flagship of the Home Fleet. Iron Duke replaced her as flagship in early 1914 and she was assigned to the 1st Battle Squadron.

The ship became part of the Grand Fleet when it was formed from parts of the Home Fleets shortly after the beginning of the First World War in August 1914. Aside from participating in the Battle of Jutland in May 1916, and the inconclusive action of 19 August several months later, her service during the war generally consisted of routine patrols and training in the North Sea. Neptune was deemed obsolete after the war and was reduced to reserve before being sold for scrap in 1922 and subsequently broken up.

The launch of Dreadnought in 1906 precipitated a naval arms race when Germany accelerated its naval construction plans in response. Despite this sudden expansion of another nation's fleet, the British Admiralty felt secure in the knowledge that Germany would have only four modern capital ships in commission by 1910, while the Royal Navy would have eleven. Accordingly, they proposed the construction of only a single battleship and a battlecruiser in the 1908–1909 naval budget that they sent to the government in December. The Liberals, committed to reducing military expenditures and increasing social welfare spending, wished to cut the budget by £1,340,000 below the previous year's budget, but were ultimately persuaded not to do so after the Prime Minister, Sir Henry Campbell-Bannerman, was thoroughly briefed on each part of the budget in February 1908. The debates over the budget in March were heated; critics were dissatisfied with the number of ships being built, arguing that the Government was too complacent about the superiority of the Royal Navy over the Imperial German Navy, but were satisfied when H. H. Asquith, Chancellor of the Exchequer, filling in for the fatally ill Prime Minister, announced that the Government was prepared to build as many dreadnoughts as required to negate any possible German superiority as of the end of 1911.

Neptune was an improved version of the preceding St Vincent class with additional armour and the armament rearranged for greater efficiency. She was the first British dreadnought that differed in her gun turret layout from Dreadnought. Unlike the earlier ships, her wing turrets were staggered "en echelon" so all five turrets could shoot on the broadside, although in practice the blast damage to the superstructure and boats made this impractical except in an emergency. This was done to match the 10-gun broadside of the latest foreign designs like the American Delaware class, although the all-centreline turret configuration of the American ships eliminated the blast problems that compromised the effectiveness of the "en echelon" arrangement. Neptune was also the first British dreadnought to be equipped with superfiring turrets, in an effort to shorten the ship and reduce costs. A further saving in length was achieved by siting the ship's boats on girders over the two wing turrets to reduce the length of the vessel. The drawback to this arrangement was that if the girders were damaged during combat, they could fall onto the turrets, immobilising them. The bridge was also situated above the conning tower, which similarly risked being obscured if the bridge collapsed.

Neptune had an overall length of 546 feet (166.4 m), a beam of 85 feet (25.9 m), and a deep draught of 28 feet 6 inches (8.7 m). She displaced 19,680 long tons (20,000 t) at normal load and 23,123 long tons (23,494 t) at deep load. The ship had a metacentric height of 6.5 feet (2.0 m) at deep load. Her crew numbered about 756 officers and ratings upon completion and 813 in 1914.

The ship was powered by two sets of Parsons direct-drive steam turbines, each of which was housed in a separate engine room. The outer propeller shafts were coupled to the high-pressure turbines and these exhausted into low-pressure turbines which drove the inner shafts. The turbines used steam from eighteen Yarrow boilers at a working pressure of 235 psi (1,620 kPa; 17 kgf/cm²). They were rated at 25,000 shaft horsepower (19,000 kW) and gave Neptune a maximum speed of 21 knots (39 km/h; 24 mph). She carried a maximum of 2,710 long tons (2,753 t) of coal and an additional 790 long tons (803 t) of fuel oil that was sprayed on the coal to increase its burn rate. This gave her a range of 6,330 nautical miles (11,720 km; 7,280 mi) at a cruising speed of 10 knots (19 km/h; 12 mph).

Neptune was equipped with ten 50-calibre breech-loading (BL) 12-inch (305 mm) Mark XI guns in five hydraulically powered twin-gun turrets, three along the centreline and the remaining two as wing turrets. The centreline turrets were designated 'A', 'X' and 'Y', from front to rear, and the port and starboard wing turrets were 'P' and 'Q' respectively. The guns had a maximum elevation of +20° which gave them a range of 21,200 yards (19,385 m). They fired 850-pound (386 kg) projectiles at a muzzle velocity of 2,825 ft/s (861 m/s) at a rate of two rounds per minute. The ship carried 100 shells per gun.

Neptune was the first British dreadnought with her secondary armament of sixteen 50-calibre BL four-inch (102 mm) Mark VII guns installed in unshielded single mounts in the superstructure. This change was made to address the problems that plagued the turret-roof locations used in earlier battleships. Notably, the exposed guns were difficult to work when the main armament was in action as was replenishing their ammunition. Furthermore, the guns could not be centrally controlled to coordinate fire at the most dangerous targets. The guns had a maximum elevation of +15° which gave them a range of 11,400 yd (10,424 m). They fired 31-pound (14.1 kg) projectiles at a muzzle velocity of 2,821 ft/s (860 m/s). They were provided with 150 rounds per gun. Four quick-firing three-pounder (1.9 in (47 mm)) Hotchkiss saluting guns were also carried. The ship was equipped with three 18-inch (450 mm) submerged torpedo tubes, one on each broadside and another in the stern, for which 18 torpedoes were provided.