The Neptune, 80 guns, was built in 1803 at Toulon, and was commissioned just in time to join the abortive attempt to break the British blockade of the port in 1804. Early in 1805 she sailed under Captain Esprit-Tranquille Maistral as part of the fleet under Admiral Villeneuve that finally broke out and cruised to the West Indies with Nelson in pursuit. Achieving little there, the fleet returned, engaging with Calder's fleet at the Battle of Cape Finisterre, where Neptune was relatively unscathed, taking casualties of just three dead and nine wounded. The French admiral shadowed the retreating British fleet for two days, but made no attempt to join battle, and instead sailed to La Coruna to await further instructions. Ordered by Napoleon to sail to Brest and Boulogne to accompany the invading armée d'Angleterre, Villeneuve instead sailed for Cadiz, where the combined fleets were blockaded by Nelson. Refusing all attempts to order him out, Villeneuve was finally prompted to leave Cadiz by news of his supersession--Napoleon described him as "not possessing the strength of character to command a frigate; he lacks determination and has no moral courage."
In the ensuing battle, Nelson's Victory at the head of the British weather column broke the line of the combined fleets just ahead of the Neptune, who fired into her with a broadside from her larboard battery. During the skirmishing that followed the Neptune engaged with the Royal Sovereign, Temeraire (who she rendered unmanageable), Belle-Isle, and Polyphemus. As the outcome of the battle became clear she joined the retreat to Cadiz having sustained only a relatively minor amount of damage, and only 15 killed and 39 wounded. On 23 October the Neptune joined the sortie led by Captain Julien Cosmao in an effort to win back some of the British prize ships, but bad weather disrupted the attempt, and she ended up being towed back into Cadiz. There she remained until the Spanish joined the war against the French in 1808, and the French fleet was taken into Spanish hands, Neptune being renamed Neptuno in replacement for the ship of that name lost at Trafalgar. She was broken up in 1820.