Halo 2 is a 2004 first-person shooter game developed by Bungie and published by Microsoft Game Studios. Released for the Xbox, the game is the second installment in the Halo franchise and the sequel to 2001's critically acclaimed Halo: Combat Evolved. The game features a new game engine, added weapons and vehicles, and new multiplayer maps. The game shipped with global multiplayer matchmaking via Microsoft's Xbox Live service. In Halo 2's campaign story, the player assumes the roles of both the human Master Chief and the alien Arbiter in a 26th-century conflict between the human United Nations Space Command, the genocidal Covenant, and the parasitic Flood.

After the success of Halo: Combat Evolved, a sequel was expected and highly anticipated. Bungie found inspiration in plot points and gameplay elements that had been left out of their first game, including online multiplayer. A troubled development and time constraints forced cuts to the scope of the game, including the wholesale removal of a more ambitious multiplayer mode, and a cliffhanger ending to the game's campaign mode. Among Halo 2's marketing was an early alternate reality game called "I Love Bees" that involved players solving real-world puzzles. Bungie supported the game after release with new multiplayer maps and updates to address cheating and glitches. The game was followed by a sequel, Halo 3, in September 2007.

Halo 2 was a commercial and critical success and is often listed as one of the greatest video games of all time. The game became the most popular title on Xbox Live, holding that rank until the release of Gears of War for the Xbox 360 nearly two years later. Halo 2 is the best-selling first-generation Xbox game, with more than 8 million copies sold worldwide. The game received critical acclaim, with the multiplayer lauded; in comparison, the campaign and its cliffhanger ending was divisive. The game's online component was highly influential and cemented many features in future games and online services, including matchmaking, lobbies, and clans. Halo 2's marketing heralded the beginnings of video games as blockbuster media. A port of the game for Windows Vista was released in 2007, followed by a high-definition remastered version in 2014 as part of Halo: The Master Chief Collection.