HMS Diadem was a 64-gun third rate ship of the line of the Royal Navy, launched on 19 December 1782 at Chatham. She participated in the Battle of Cape St Vincent in 1797. In 1798 she was converted to serve as a troopship. In 1800 under the command of Post Captain Sir Thomas Livingstone she was employed in the expedition to Quiberon and Belle Île under Sir Edward Pellew, subsequently she was employed in the expedition to Cádiz under Admiral Lord Keith.