The 18-pounder long gun was an intermediary calibre piece of naval artillery mounted on warships of the Age of Sail.

It was a heavy calibre for early ships of the line, arming, for instance, the main batteries of Couronne in 1636.

From the late 18th century, the French Navy used the 18-pounder in three capacities: as the main gun on frigates, as the battery on the upper gundeck of two-deckers, and lastly on the top deck of three-deckers.

Under Louis XVI, from 1779, the 18-pounder gradually became the standard calibre for frigates, starting with the Hébé class.

These frigates were built on standard patterns designed by Jacques-Noël Sané, carrying 26, and later 28 main guns, complemented with smaller pieces on the forecastle.

A 74-gun would carry thirty 18-pounders; this lighter secondary battery added firepower to the ship without raising the centre of gravity too much.

In rough weather, vessels often could not use their main battery lest water enter through the gun-ports, and the secondary battery then became the vessel's main armament; for example, the Droits de l'Homme was effectively reduced to the firepower of a frigate when she fought the action of 13 January 1797 in stormy weather, leading to her destruction at the hand of two British frigates that would normally not have been a match for her; in the opposite case, during the Glorious First of June, Vengeur du Peuple used her main batteries but became unmanageable and sank after taking in water from her lower gun-ports, whose covers had been ripped off in a collision with HMS Brunswick.

Subsequent mentions of the 18-pounder describe what are likely 3 types of 18-pounder, whose dimensions changed little through the 1700s: The gun of 9 ft appears in a diagram from about 1735, where its weight was listed as 41 hundredweight 1 quarter 8 pounds.

3 guns of 9 feet length from the reign of George II (1727-1760) currently survive at the Gut of Digby in Nova Scotia.

In his discussion of the single-ship action in which the French frigate Piémontaise captured the East Indiaman Warren Hastings on 11 June 1805, the naval historian William James compared the 18-pounder medium guns on Warren Hastings with the 18-pounder long guns that the British Royal Navy used.