"Neptuno" was built at Ferrol and launched in 1795. She entered service in time to support an attempt to unite with a French force and land troops in England, but the Spanish fleet under Admiral José de Córdoba y Ramos was intercepted and engaged by a British fleet under Sir John Jervis. "Neptuno" did not take part in the battle, having been sent into port beforehand. Several years later she was in a Spanish port when the combined Franco-Spanish fleet under Vice-Admiral Pierre-Charles Villeneuve arrived, having sailed to the West Indies and back, and been engaged by a British fleet in the Battle of Cape Finisterre. "Neptuno" joined the fleet in her attempt to reach Brest, but the plan to join with another French fleet failed and "Neptuno" ended up with the rest of the fleet, blockaded in Cadiz by a British fleet under Lord Nelson.