HMS Diadem was a 64-gun third rate ship of the line of the Royal Navy, launched on 19 December 1782 at Chatham.

[2] She participated in the Battle of Cape St Vincent in 1797 under Captain George Henry Towry.

Because Diadem served in the navy's Egyptian campaign between 8 March 1801 and 2 September, her officers and crew qualified for the clasp "Egypt" to the Naval General Service Medal that the Admiralty authorised in 1850 to all surviving claimants.

[Note 1] Between April and July 1810 Diadem was at Chatham being fitted for service as a troopship of 28 guns.

[5] She then spent some time working with the Spanish anti-French forces on the north coast of Spain.