Halo 3 is a 2007 first-person shooter video game developed by Bungie for the Xbox 360 console. The third installment in the Halo franchise following Halo: Combat Evolved (2001) and Halo 2 (2004), the game's story centers on the interstellar war between 26th-century humanity, a collection of alien races known as the Covenant, and the alien parasite known as the Flood. The player assumes the role of the Master Chief, a cybernetically enhanced supersoldier, as he battles the Covenant and the Flood. In cooperative play, other human players assume the role of allied alien soldiers. The game features vehicles, weapons, and gameplay elements familiar and new to the series, as well as the addition of saved gameplay films, file sharing, and the Forge map editor—a utility which allows the player to perform modifications to multiplayer levels.

Halo 2 had originally been intended to wrap up the story begun with Combat Evolved, but development difficulties led to a cliffhanger ending. Bungie began developing Halo 3 shortly after Halo 2 shipped. The game was officially announced at E3 2006, and its release was preceded by a multiplayer beta open to select players who purchased the Xbox 360 game Crackdown. Microsoft spent $40 million on marketing the game, in an effort to sell more game consoles and broaden the appeal of the game beyond the established Halo fanbase. Marketing included cross-promotions and an alternate reality game.

Halo 3 was released on September 25 and grossed US$170 million on its first day of release, rising to $300 million in its first week. The game sold in excess of 14.5 million copies and was the best-selling video game of 2007 in the United States. More than one million people played Halo 3 on Xbox Live in the first twenty hours. Overall, the game was well received by critics, with the Forge and multiplayer offerings singled out as strong features; however, some reviewers criticized single-player aspects, especially the plot and campaign layout. Halo 3 is frequently listed as one of the greatest video games of all time. A sequel, Halo 4, released in November 2012, was developed by 343 Industries. Halo 3 was re-released as part of Halo: The Master Chief Collection for the Xbox One in November 2014 and for Windows on July 14, 2020.

Halo 3 is a shooter game where players primarily experience gameplay from a first-person perspective. Much of the gameplay takes place on foot, but also includes segments focused on vehicular combat. Combat focuses on the use of weapons, grenades, and melee attacks, which are available to a player in most situations. Players may dual-wield some weapons for additional firepower, forgoing the use of grenades and melee attacks. Unlike previous installments, the player's secondary weapon is visible on their player model, holstered or slung across the player's back. Halo 3 introduces support weapons, cumbersome two-handed weapons that slow the player when carried, but offer greatly increased firepower in return.

In addition to weapons, Halo 3 contains a new class of gear called equipment; these items have various effects, ranging from defensive screens to shield regeneration and flares. Only one piece of equipment can be carried at a time. The game's vehicular component has been expanded with new drivable and AI-only vehicles.

Halo 3 contains non-gameplay additions, such as Forge, a map-editing tool. Forge enables players to insert and remove game objects, such as weapons, crates, and vehicles into existing multiplayer maps. Almost all weapons, vehicles, and interactive objects can be placed and moved on maps with Forge. Players can enter Forge games and edit and manipulate objects in real time. A budget limits the number of objects that can be placed. Players may also save up to 100 films of gameplay to their Xbox 360's hard drive, viewing the action from any angle and at different speeds. Halo 3 offers a form of file sharing, where items such as saved films, screenshots, and custom variants can all be uploaded to Bungie's official website. Anyone can browse user created content that has been uploaded to Bungie's website and tag it to automatically download to their console next time they sign into Xbox Live on Halo 3.

Halo 3's story or campaign mode can be played alone or cooperatively with up to three other players via Xbox Live or System Link. Instead of each player being an identical character in cooperative play, as in previous Halo games, the first player plays as Master Chief, the second player as the Arbiter, and the final two players controlling the Covenant Elites N'tho 'Sraom and Usze 'Taham. Each player has identical abilities, although their starting weapons vary. Hidden skulls found on each level cause changes to the gameplay when enabled, such as giving the enemies extra health, changing in-game dialogue, or modifying AI behavior. These skulls, as well as the difficulty level and the speed at which the level is completed, provide multipliers to the total score. Players are awarded gamerscore points for unlocking Achievements by reaching a certain score in each level.

Local area network or Xbox Live supports up to sixteen players in multiplayer matches, with game modes including variations of deathmatch and Capture the Flag. Players must actively seek out other players through their Xbox Live Friends list, using the party invite system, or the LAN search feature to play multiplayer matches with their own custom rules and customized maps. If they are connected to Xbox Live however, a player can choose to have the game decide for them the exact rules and map to play on, as well as finding additional people to play against or with, using the "Matchmaking" system (the automated grouping of players of similar skill). A player will decide from a selection of developer designed "playlists" which each contain a certain way to experience the game.