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.

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.

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.

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 they 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.

[1] Neptune was an improved version of the preceding St Vincent class with additional armour and the armament rearranged for greater efficiency.

Neptune was also the first British dreadnought to be equipped with superfiring turrets, in an effort to shorten the ship and reduce costs.

[5] The ship was powered by two sets of Parsons direct-drive steam turbines, each of which was housed in a separate engine room.

[5] 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.

[13] Neptune was the first British dreadnought to be built with a gunnery director, albeit an experimental prototype designed by Vice-Admiral Sir Percy Scott.

[16] Furthermore, the ship was fitted with Mark I Dreyer Fire-control Tables by early 1916 in each transmission station that combined the functions of the Dumaresq and the range clock.

[8] The ship was commissioned on 19 January 1911 for trials with Scott's experimental gunnery director that lasted until 11 March and were witnessed by Rear-Admiral Sir John Jellicoe, commander of the Atlantic Fleet.

Neptune became a private ship on 10 March 1914 when she was replaced by Iron Duke as the flagship of the Home Fleet and rejoined the 1st BS.

On the evening of 22 November 1914, the Grand Fleet conducted a fruitless sweep in the southern half of the North Sea; Neptune stood with the main body in support of Vice-Admiral David Beatty's 1st Battlecruiser Squadron.

[Note 2] Another such cruise took place on 16–19 March; while returning home after the conclusion of the exercises, Neptune was unsuccessfully attacked by the German submarine SM U-29.

On 2–5 September, the fleet went on another cruise in the northern end of the North Sea, conducting gunnery drills, and spent the rest of the month performing numerous training exercises.

Another sweep began on 6 March, but had to be abandoned the following day as the weather grew too severe for the escorting destroyers.

On the night of 25 March, Neptune and the rest of the fleet sailed from Scapa Flow to support Beatty's battlecruisers and other light forces raiding the German Zeppelin base at Tondern.

On 21 April, the Grand Fleet conducted a demonstration off Horns Reef to distract the Germans while the Imperial Russian Navy relaid its defensive minefields in the Baltic Sea.

[33] During the night of 22/23 April, Neptune was accidentally rammed by the neutral merchant ship SS Needvaal in thick fog, but the battleship was only lightly damaged.

[35] On 31 May, Neptune, under the command of Captain Vivian Bernard, was assigned to the 5th Division of the 1st BS and was the nineteenth ship from the head of the battle line after deployment.

[22] During the first stage of the general engagement, the ship fired two salvos from her main guns at a barely visible battleship at 18:40.

After the turn the ships of the 1st BS were the closest ones to the Germans and, at approximately 19:10, she fired four salvos at the battlecruiser SMS Derfflinger, claiming two hits, although neither can be confirmed.

Shortly afterwards, the ship fired her main and secondary guns at enemy destroyers without result and then had to turn away to dodge three torpedoes.

[37] On 22 April 1918, the High Seas Fleet sailed north for the last time in an unsuccessful attempt to intercept a convoy to Norway, and had to turn back two days later after the battlecruiser SMS Moltke suffered engine damage.

[38] Neptune was present at Rosyth when the German fleet surrendered on 21 November and was reduced to reserve there on 1 February 1919 as she was thoroughly obsolete in comparison to the latest dreadnoughts.