Left 4 Dead 2 is a 2009 first-person shooter game developed and published by Valve. The sequel to Turtle Rock Studios's Left 4 Dead (2008) and the second game in the Left 4 Dead series, it was released for Microsoft Windows and Xbox 360 in November 2009, Mac OS X in October 2010, and Linux in July 2013.

Left 4 Dead 2 builds upon cooperatively focused gameplay and Valve's proprietary Source engine, the same game engine used in the original Left 4 Dead. Set during the aftermath of an apocalyptic pandemic, the game focuses on four new Survivors, fighting against hordes of zombies known as the Infected, who develop severe psychosis and act extremely aggressive. The Survivors must fight their way through five campaigns, interspersed with safe houses that act as checkpoints, with the goal of escape at each campaign's finale. The gameplay is procedurally altered by the "AI Director 2.0", which monitors the players' performance and adjusts the scenario to provide a dynamic challenge. Other new features include new types of Special Infected and an arsenal of melee weapons.

The game made its world premiere at E3 2009 with a trailer during the Microsoft press event. Prior to release, it received mixed critical and community reactions, and attracted an unusually high volume of controversy about its graphic content. In response, alterations were made to the cover art, but both Australia and Germany refused to rate the unmodified edition at the time of release. Despite this, the game was met with positive critical reviews, and is considered to be one of the greatest video games ever made.