Turret Defense is a custom map for Blizzard Entertainment's 1998 real-time strategy video game StarCraft.

A variation on Tower Defense, Turret Defense requires the player to fend off enormous hordes of air units using only ground-to-air defensive structures. Initially, each player of six is equipped with a single SCV and enough resources to build four Terran Missile Turrets, which give the map its name. After a short period of buiding and preparation time, the enemy moves down a pre-determined path, while the players and their Turrets attempt to thin the horde sufficiently. Placement of Turrets is critical, as there are zones of bad coverage and good coverage (one, in particular, brings the enemy into range of the player's Turrets twice), and one can go for many rounds without achieving even a single kill if one is not careful. Killing the constituents of each wave of enemy forces gives each player resources: money for a new Turret every few kills (generally five), and a special token more infrequently (perhaps twenty-five), which can later be redeemed for soldiers, or (even better), a Protoss Probe or a Zerg Drone, each of which allows the player to employ that race's defensive structure (Photon Cannons and Spore Colonies, respectively), which are stronger but more expensive. (There are also certain terrain-management requirements associated with the Zerg and its Creep.) As usual, if an enemy makes it through the entire gauntlet, there are dire consequences; depending on the map's difficulty level, picked by the red-colored player at the beginning of the game, the map may allow between 10 to 30 enemy transgressions before imposing a Game-Over.

There are many variations. In one, each of five players has a different anti-air-unit resource to employ. Player 1 uses SCVs, Player 2 uses Probes, Player 3 is given Hydralisks, Player 4 controls a group of Terran Goliaths, and Player 5 is given a Terran Ghost who can call for air strikes. In another, the map is shaped rather like a large, four-tined fork. Of the six players, four start at the tips of the tines, and two at the junctions where the tines join the 'handle' of the fork, at the tip of which is the goal zone that no enemy should be allowed to reach. (The junction players often resign of boredom before getting to ever fire a shot.) In this version, the enemy does not come in actual discreet shipments, but instead in long streams determined in enemies per minute. The first wave, for instance, consists of Zerg Overlords, which spawn at a rate of perhaps two every five seconds, and make their way very slowly down the various aisles. Once a certain amount of time has been reached, a new wave will be added, consisting of Terran Valkyies, which spawn less frequently but have a faster airspeed and more durability. However, the streams of Overlords do not cease. Likewise, the third wave (of Terran Wraiths) is simply overlaid onto the first two. This version of the map is nearly unwinnable, as eventually the number of objects in play exceeds the computer's maximum capacity, forcing it to cut out the creation of new ones--which includes, for instance, the projectiles fired by the player's Missile Turrets.