The Great Cannon is a siege unit that cannot be created during normal gameplay. It is found in the campaign "Act II: Ice" of Age of Empires III, wherein it is very strong heavy Russian artillery. In the level, players must halt and destroy Great Cannons in order to prevent them from crossing the player's starting position. The Great Cannon is pulled by two cavalry (like the Great Bombard) and has the firing sound of a ship gun. It is very effective against infantry and can only be countered effectively by cavalry in hand-to-hand combat and anti-artillery artillery.

A replacement for the Great Cannon in Age of Empires III: The WarChiefs, is the Li'l Bombard mercenary unit available from the Home City and the Saloon.