HMS Canopus was an 84-gun third rate ship of the line of the British Royal Navy. She had previously served with the French Navy as the Tonnant-class Franklin, but was captured after less than a year in service by the British fleet under Rear Admiral Horatio Nelson at the Battle of the Nile in 1798. Having served the French for less than six months from her completion in March 1798 to her capture in August 1798, she eventually served the British for 89 years.

The Tonnant class was a series of eight 80-gun ships of the line designed in 1787 by Jacques-Noël Sané, whose plans for the prototype were approved on 29 September 1787. With sixteen gunports on the lower deck on each side these were the most effective two-deckers of their era. Their broadside of 1,102 livres equated to 1,190 British pounds weight, over 50% more than the standard British 74-gun ship, and even greater than that of a British 100-gun three-decker.