Left 4 Dead is a 2008 first-person shooter game developed by Valve South and published by Valve. It was originally released for Windows and Xbox 360 in November 2008 and for Mac OS X in October 2010, and is the first title in the Left 4 Dead series. Set during the aftermath of a zombie outbreak on the East Coast of the United States, the game pits its four protagonists, dubbed the "Survivors", against hordes of the infected.

Left 4 Dead uses Valve's proprietary Source engine, with four game modes: a single-player mode in which allied characters are controlled by AI, a four-player co-op campaign mode, an eight-player online versus mode, and a four-player survival mode. In all modes, an artificial intelligence dubbed the "Director" controls level pacing and item placements in an attempt to create a dynamic experience and increase replay value.

Left 4 Dead received praise for its replay value, focus on cooperative play, and cinematic feel, although some criticized its limited level selection and lack of narrative. Considered one of the greatest video games ever made, the game won several publication awards, as well as distinctions from the Academy of Interactive Arts & Sciences and British Academy of Film and Television Arts. As was done with Team Fortress 2, Valve supplemented the game with free downloadable content. The success of the game led to the development of the sequel Left 4 Dead 2, which was released in 2009. In 2012, all Left 4 Dead campaigns were ported over to Left 4 Dead 2, with cross-platform multiplayer support between Windows and Mac versions of the game.