Call of Duty: Modern Warfare 2 is a 2009 first-person shooter game developed by Infinity Ward and published by Activision. It is the sixth installment in the Call of Duty series and the direct sequel to Call of Duty 4: Modern Warfare. It was released worldwide on November 10, 2009, for Windows, PlayStation 3, and Xbox 360. A separate version for the Nintendo DS, titled Modern Warfare: Mobilized, was also released on the same day. A version for OS X was developed by Aspyr and released in May 2014, and the Xbox 360 version was made backward compatible for the Xbox One in 2018.

The game's campaign follows Task Force 141, a multinational special forces unit commanded by Captain Soap MacTavish as they hunt Vladimir Makarov, leader of the Russian Ultranationalist party, and United States Army Rangers from the 1st Ranger Battalion who are defending the Washington, D.C. area from a Russian invasion. The game's main playable characters are Sergeant Gary "Roach" Sanderson, of the 141, and Private James Ramirez, of the Army Rangers, with Captain MacTavish becoming playable later in the campaign. The multiplayer mode was expanded upon from the previous game, with several new features and modes.

Development for the game began in 2008, when it was still known as Call of Duty 6. It uses the IW 4.0 engine, an improved version of Call of Duty 4's IW 3.0. Infinity Ward was inspired by real-life conflicts when developing the campaign mode. They initially tested the multiplayer mode by playing an in-house beta version of the game. Modern Warfare 2 was officially announced in February 2009. Teasing of the game began in March, with short trailers being released for the game and, eventually, a full reveal trailer. The multiplayer mode was revealed shortly after. Two downloadable content packs were released for it post-release, each containing five new multiplayer maps, with some being remastered maps from Call of Duty 4.

Modern Warfare 2 received universal acclaim, with praise for its campaign, multiplayer, and amount of content, although it received some criticism for its short length and a lack of innovation. The game was also subject to a controversy surrounding a playable level that had the player participate in a terrorist attack on an airport. Retrospective reviews consider it one of the best games in the series. Within 24 hours of release, the game sold approximately 4.7 million copies in North America and the United Kingdom, ending as the best-selling video game in 2009 in the US. As of 2013, the game had sold 22.7 million copies, becoming one of the best-selling PlayStation 3 video games and best-selling Xbox 360 video games. A sequel, Call of Duty: Modern Warfare 3, was released in 2011 and finishes the original Modern Warfare storyline. A remaster of the game's campaign, Call of Duty: Modern Warfare 2 Campaign Remastered, was released on the PlayStation 4 in March 2020 and April 2020 for Windows and Xbox One.

Call of Duty: Modern Warfare 2 is a first person shooter, and its gameplay revolves around fast-paced gunfights against enemy combatants. The player controls a soldier who can perform several actions, including jump, sprint, crouch, lay prone, and aim down their gun's iron sights. When the player is shot by an enemy, blood will splatter their heads-up display (HUD), indicating that they have taken damage; if the player avoids gunfire by taking cover, their health will recover automatically within some time limit. The HUD also displays other information, such as a compass, a mini-map, and the player's current ammunition count. The game features traditional guns, including assault rifles, shotguns, handguns, and sniper rifles. The player will be given specific guns at the beginning of each level, but may switch them out with another gun they find (either among dead bodies or near supply unit). Some guns have attachments, such as suppressors, and heartbeat sensors. The player can use grenades and flashbangs when faced with a large group of enemies, as well as a knife for close quarters combat. In some levels, the player will be given special equipment, such as night vision goggles, or a laser designator.

Call of Duty: Modern Warfare 2 has three different game modes: Campaign, Spec Ops, and Multiplayer. Campaign is a single-player mode where the player completes eighteen levels connected by an overarching plot. Each level features a series of objectives to fulfill, and the player will often switch characters between levels. If the player dies during a level, they will respawn at the most recent checkpoint. Levels can be played on one of four difficulties, and each level can be replayed after it has been completed. Spec Ops mode features twenty-three additional levels that can be played individually or cooperatively with a partner. These levels provide specific challenges, such as defusing three bombs within a short period of time. If one player is shot down while playing cooperatively, they will begin crawling, and can shoot enemies with a handgun. If they are not revived by the other player, then they will die and fail the level. There are five tiers of Spec Ops levels, with each tier harder than the previous. Only the first tier is available from the beginning, as later tiers can be unlocked with enough stars. The player earns stars by completing the levels on one of three difficulties, with the number of stars earned corresponding to the difficulty chosen.

Multiplayer mode allows players to compete against each other in team-based and deathmatch-based game types on various maps. Each game type has an objective that requires unique strategies to complete. If the player kills three or more players in a row without dying, they achieve a "killstreak", which gives the player a tactical advantage during a match. These include a Predator missile, a sentry gun, and a tactical nuke. Alternatively, if the player dies several times without a kill, they will be rewarded with a "deathstreak" bonus, which evens the match for the player. A match ends when either a team or player has reached a predefined number of points, or the allotted time expires in which case the team or player with the most points wins. The player's performance in multiplayer is tracked with experience points, which can be earned by killing opposing players, completing objectives, or by completing a match. As the player gains experience, they advance in level, unlocking new weapons. The player will also unlock perks, which modify gameplay elements such as unlimited sprint and increased bullet damage.

In 2016, Ultranationalists seize control of Russia and Imran Zakhaev becomes a martyr. Diplomatic relations with the United States plummet, while Zakhaev's protégé, Vladimir Makarov, commits terrorist attacks against the West.