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 Microsoft Windows, the 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 macOS 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 multi-national special forces unit commanded by Captain Soap MacTavish as they hunt Vladimir Makarov, leader of the Russian Ultranationalist party, and U.S. Army Rangers from 1st Battalion/75th Ranger Regiment 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. An expansive multiplayer mode is featured in the game, with several new features and modes that were not seen in its predecessor.

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. Within 24 hours of release, the game sold approximately 4.7 million copies in North America and the United Kingdom. As of 2013, the game had sold 22.7 million copies. In addition to its release, a comic book series based on character Simon "Ghost" Riley was also produced, titled Modern Warfare 2: Ghost. Despite the game's success, it was subject to some controversies, with one surrounding a playable level that had the player carry out a terror attack on an airport. 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 in March 2020 on the PlayStation 4 and on Microsoft Windows and Xbox One in April 2020.