Halo 3 is a first-person shooter developed by Bungie Studios exclusively for the Xbox 360. The game is the third title in the Halo series and ends the story arc begun in Halo: Combat Evolved and continued with Halo 2. The game was released on September 25, 2007 in New Zealand (which due to its midnight release made it first available for official retail), Australia, Singapore, India, Mexico, Canada, Brazil, and the United States; September 26, 2007 in Europe; and September 27, 2007 in Japan. GameSpot reported that 4.2 million units of Halo 3 were in retail outlets on September 24, 2007, a day before official release, a world record volume release. Halo 3 also holds the record for the highest grossing opening day in entertainment history, making US$170 million in its first 24 hours.

The game features new vehicles, weapons, and gameplay features not present in the previous titles of the series. Halo 3 focuses on the interstellar war between 26th century humanity and a collection of alien races known as the Covenant, who after a decades-long war have begun the invasion of Earth. The player assumes the role of the Master Chief, a cybernetically enhanced supersoldier, as he wages war in defense of humanity.

The gameplay of Halo 3 largely builds upon the previous iterations of the franchise. It is a first-person shooter that follows the character Master Chief and his struggle against the Covenant and the Flood. The game's action takes place largely on foot, but also includes segments focused on vehicular combat.

The balance of weapons and objects in the game was adjusted to better adhere to what Bungie Multiplayer Designer Lars Bakken describes as the "Golden Triangle of Halo". These are "weapons, grenades, and melee", all three of which are normally available to a player. Halo 3 also contains the ability to dual-wield, where a player forgoes grenades for a pair of weapons. All of the weapons available in previous installments of the series, except for the Brute Plasma Rifle, return with minor cosmetic and power alterations.

 Unlike previous installments, all weapons a player is carrying are visible; weapons not in use are holstered or slung across the player's back. Halo 3 also introduced "support weapons", which are exceptionally large, powerful and cumbersome two-handed weapons such as machine gun turrets and flamethrowers. These weapons drastically limit the player's normal combat options and slow them significantly. In return, they offer greatly increased firepower. In addition to new weapons, Halo 3 added a new class of usable items called Equipment. These items have various effects and functions, ranging from defensive effects (Bubble Shield and Regenerator), or objects which blind or confuse the enemy (Flare^[ and Radar Jammer), to ones which can actually harm and kill (Power Drainer and Tripmine). A player can only carry one of these items at a time. The majority are placed on the battlefield, where they can be destroyed by weapons fire or explosives. Most equipment also has a limited duration after being used.

Halo 3 also features a strong vehicular component, with new vehicles never before seen added in the third installment of the series. Most vehicles can be used by the player, but some are A.I.-controlled. Certain vehicles usable in campaign are not available in multiplayer due to balance concerns.

Halo 3's campaign does not begin right where Halo 2 left off. Instead, the comic mini-series, Halo: Uprising, bridges the story gap between the ending of Halo 2 and the beginning of Halo 3.^[26]

The game begins with the Master Chief entering Earth's atmosphere and falling to the ground. Sergeant Major Avery Johnson and a squad of men discover him and begin to check his vitals. The Master Chief, along with Sergeant Major Johnson and the Arbiter, make their way through the jungle towards a UNSC outpost. During this time, the Master Chief seems to experience disturbing broken transmissions from Cortana, who he left behind on the Flood-controlled Covenant capital city/ship, High Charity. Shortly after they reach their destination, the Covenant mounts an attack on the facility. After a brief struggle, in which the outpost is lost, the Master Chief receives orders from Commander Keyes and Lord Hood to clear a way into the African city of Voi, and destroy all anti-air Covenant defenses in preparation for an all-out assault on the Prophet of Truth and the artifact he is uncovering.

The remains of Earth's fleet uses the opening caused by the ground attack to mount an offensive against Truth's ship, but the Prophet activates the buried artifact and creates an enormous, stable slipspace portal; he and his followers enter. As the human ships attempt to recover from the shock wave, a Flood-controlled Covenant ship arrives via slipspace and crash-lands nearby. Covenant Elite forces, now allied with humanity after a civil war, arrive on Earth, and help glass Flood-infected areas of Earth, neutralizing the threat. The Master Chief, Arbiter, Elites, and Keyes decide to follow Truth through the slipspace portal. Joining them is the Forerunner construct 343 Guilty Spark, who decides to help.

After going through the slipspace portal, the UNSC and Elites find an immense artificial structure– the Ark– which is well outside the Milky Way galaxy, possibly outwith the relative universe, given the fact that it would appear beyond the reach of a normal slipspace jump. Here, Truth can activate all the Halos and purge the galaxy of all sentient life; the Chief and company are determined to stop him. Guided by Guilty Spark, the Chief and the Arbiter make their way towards the control room of the station. During their journey, a Flood-controlled High Charity arrives by slipspace, and Flood begin infesting the Ark. Johnson is captured by Truth in an effort to activate the rings, and despite the arrival of Commander Keyes, the Ark is activated. The Flood intelligence Gravemind forges a truce with the Chief and Arbiter in an effort to stop the firing. Keyes is killed by Truth, who in turn is killed by the Arbiter, but the installation's firing is halted. Gravemind quickly turns on the Chief and Arbiter, but Johnson flies away while the Chief and Arbiter fight their way out. They discover that the Ark has constructed a new Halo structure to replace the one the Master Chief destroyed in Halo: Combat Evolved. The Chief decides to activate the new ring to destroy the local Flood and the Gravemind while sparing the galaxy at large. He first goes to High Charity to recover Cortana from Gravemind. After rescuing her, Cortana reveals that she still has the activation key, the "Index", from the first Halo, which can be used on the new ring. The Master Chief overloads a main reactor and destroys High Charity, apparently killing Gravemind in the process.

Arriving on Halo, it is discovered that Gravemind is still alive and attempting to reconstruct himself. The Chief, the Arbiter, and Sergeant Major Johnson make their way to the control room, where they attempt to fire the main weapon. Guilty Spark explains the the ring is not quite completed, and that a premature activation will destroy it and the Ark. When Johnson ignores his warning, Guilty Spark kills him. The Master Chief then destroys Guilty Spark, activates the ring, and barely manages to escape with the Arbiter and Cortana to Forward Unto Dawn, a UNSC ship.

Back on Earth, the front half of Forward Unto Dawn crashes into the ocean, and workers cut the Arbiter out of the wreckage. A memorial service is held for the fallen heroes of the war which includes the Master Chief as he, along with Cortana, apparently did not make it through the slipspace portal with the Arbiter. After the memorial service, the Arbiter boards his ship and departs for his home planet, where the Elites are finally free of the Prophet's hegemony. The game then ends with a close-up of the the number "117" (The Master Chief's UNSC given Spartan number) etched into the side of the Covenant War memorial statue.

If the player waits through the closing credits, a scene reveals that the Master Chief and Cortana have in fact survived in the rear section of the ship. Cortana explains that the slipspace portal became unstable from the catastrophic Halo activation, and they are drifting helplessly through space. Cortana activates a distress beacon, noting it might be years before they are rescued. As the Master Chief climbs into a cryo-tube to await rescue, Cortana tells the Master Chief that she will miss him. The Master Chief responds by saying "Wake me when you need me" and proceeds to close the cryo-tube. If the game is completed on the highest difficulty level, the scene continues and the severed section of the Dawn is seen drifting towards what resembles the night side of a planet covered with a network of city lights.