Half-Life: Decay is an expansion pack for Valve Software's science fiction, first-person shooter video game Half-Life. Developed by Gearbox Software and published by Sierra Entertainment, Decay was released as part of the PlayStation 2 version of Half-Life. It is the third expansion pack for Half-Life, and like its predecessors, Decay returns to the setting and timeline of the original story, albeit portraying the story from the viewpoint of a different set of protagonists: two scientists working in the Black Mesa Research Facility. Decay is a cooperative multiplayer game, designed to be played by two people working together to pass through the game's levels.

In September 2008, a group of Ukrainian developers unofficially ported the game to Windows as a modification of the PC version of Half-Life.