"Thunderer" was assigned to the fleet under Admiral Robert Calder, tasked with intercepting the Franco-Spanish fleet under Vice-Admiral Pierre-Charles Villeneuve as it returned from the Caribbean. Calder intercepted the combined fleet and brought them to battle on 22 July 1805. "Thunderer" was involved in the fighting as the second to last ship in Calder's line, and had seven men killed and eleven wounded. "Thunderer", and several of the frigates had drifted some six miles distant of the admiral on the morning of 23 July, and took no part in the second indecisive clash of the battle. The fleets then parted, with Calder's force sailing to the north with the prizes, where they met the fleet arriving from the West Indies under Lord Nelson. After sending several ships, including "Thunderer", into port, Calder sailed south to join Vice-Admiral Cuthbert Collingwood off Cadiz.