Nine ships and a naval base of the Royal Navy have been named HMS Neptune after the Roman god of the ocean:
HMS Neptune was a 90-gun second rate ship of the line of the Royal Navy, built at Portsmouth Dockyard to the draught specified by the 1745 Establishment as amended in 1750, and launched on 17 July 1757.
The Neptune was the flagship for Vice-Admiral Charles Knowles in 1757. One of Neptune's midshipmen at this time was John Hunter, later to become an admiral and the second Governor of New South Wales.
Neptune was converted to serve as a sheer hulk in 1784, and continued in this role until she was broken up in 1816.
Neptune has been identified as the subject of a 1764 prize-winning painting by Liverpool marine artist Richard Wright, subsequently engraved by William Woollett entitled The Fishery.
HMS Neptune was a Royal Navy dreadnought battleship, intended to be the lead ship of three Neptune-class battleships, but the subsequent two ships had slightly thicker belt armour and were reclassified as the Colossus class.
She was the first Royal Navy battleship that differed in her gun turret layout from Dreadnought. She had two wing turrets staggered en echelon so that all five turrets could shoot in broadside, although in practice the blast damage to the superstructure and boats made this impractical except in an emergency.
The ship was also equipped with superfiring rear turrets; arranged so that one would fire over the other when shooting towards the stern. She was the first Royal Navy ship to have a superfiring main armament (the U.S. battleship USS South Carolina, launched in 1908, was the first battleship anywhere to have superfiring main turrets). However, the upper of the two turrets could not fire within 30 degrees of the stern without the lower turret being damaged by blast through its sighting hoods.