The 18-pounder long gun was an intermediary calibre piece of naval artillery mounted on warships of the Age of Sail. They were used as main guns on the most typical frigates of the early 19th century, on the second deck of third-rate ships of the line, and even on the third deck of late first-rate ships of the line.

Naval artillery in the Age of Sail encompasses the period of roughly 1571–1862: when large, sail-powered wooden naval warships dominated the high seas, mounting a large variety of types and sizes of cannon as their main armament. By modern standards, these cannon were extremely inefficient, difficult to load, and short ranged. These characteristics, along with the handling and seamanship of the ships that mounted them, defined the environment in which the naval tactics in the Age of Sail developed.