HMS Diadem was a 64-gun third rate ship of the line

Ship of the line

A ship of the line was a type of naval warship constructed from the 17th through the mid-19th century to take part in the naval tactic known as the line of battle, in which two columns of opposing warships would manoeuvre to bring the greatest weight of broadside guns to bear...

 of the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

, launched on 19 December 1782 at Chatham

Chatham Dockyard

Chatham Dockyard, located on the River Medway and of which two-thirds is in Gillingham and one third in Chatham, Kent, England, came into existence at the time when, following the Reformation, relations with the Catholic countries of Europe had worsened, leading to a requirement for additional...

. She participated in the Battle of Cape St Vincent in 1797.

In 1798 she was converted to serve as a troopship

Troopship

A troopship is a ship used to carry soldiers, either in peacetime or wartime...

. In 1800 under the command of Post Captain Sir Thomas Livingstone she was employed in the expedition to Quiberon

Quiberon

Quiberon is a commune in the Morbihan department in Brittany in north-western France.It is situated on the southern part of the Quiberon peninsula, the northern part being the commune of Saint-Pierre-Quiberon...

 and Belle Île

Belle Île

Belle-Île or Belle-Île-en-Mer is a French island off the coast of Brittany in the département of Morbihan, and the largest of Brittany's islands. It is 14 km from the Quiberon peninsula.Administratively, the island forms a canton: the canton of Belle-Île...

 under Sir Edward Pellew, subsequently she was employed in the expedition to Cádiz

Cádiz

Cadiz is a city and port in southwestern Spain. It is the capital of the homonymous province, one of eight which make up the autonomous community of Andalusia....

 under Admiral Lord Keith.

Diadem was broken up in September 1832.