Neptune was a Bucentaure-class 80-gun ship of the line of the French Navy. Built during the last years of the French Revolutionary Wars she was launched at the beginning of the Napoleonic Wars. Her brief career with the French included several major battles, though she spent the last 12 years of her life under the Spanish flag.

In the Battle of Cape Finisterre off Galicia, Spain, the British fleet under Admiral Robert Calder fought an indecisive naval battle against the combined Franco-Spanish fleet which was returning from the West Indies. In the ensuing battle the British captured two Spanish ships of the line, but failed to prevent the joining of French Admiral Pierre de Villeneuve's fleet to the squadron of Ferrol and to strike the shattering blow that would have freed Great Britain from the danger of an invasion. Calder was later court-martialled and severely reprimanded for his failure and for avoiding the renewal of the engagement on 23 and 24 July. At the same time, in the aftermath Villeneuve elected not to continue on to Brest, where his fleet could have joined with other French ships to clear the English Channel for an invasion of Great Britain.