Bloons Tower Defense is a series of tower defense games under the Bloons series created and produced by Ninja Kiwi. The game was initially developed as a browser game, built upon the Adobe Flash platform and released in mid 2007. Later games in the series expanded to support various mobile platforms, including Android, iOS, Windows Phone, PlayStation Portable, Nintendo DSi, Windows, Linux and MacOS. Games in the Bloons series older than Bloons TD 6 are available through the Ninja Kiwi Archive on Steam.

Tower defense (TD) is a subgenre of strategy games where the goal is to defend a player's territories or possessions by obstructing the enemy attackers or by stopping enemies from reaching the exits, usually achieved by placing defensive structures on or along their path of attack. This typically means building a variety of different structures that serve to automatically block, impede, attack or destroy enemies. Tower defense is seen as a subgenre of real-time strategy video games, due to its real-time origins, even though many modern tower defense games include aspects of turn-based strategy. Strategic choice and positioning of defensive elements is an essential strategy of the genre.