Half-Life 2: Episode One is the first of a planned trilogy of expansion packs/episodes for the 2004 first-person shooter game, Half-Life 2. The episode takes place immediately after the end of Half-Life 2, in the war-torn setting of City 17. The player is forced to deal with the effects of their actions during the main game. The episode is a stand-alone game; while a continuation of Half-Life 2, it does not require the original game to be installed or registered to a user's Steam account to play. It takes advantage of several major upgrades to the Source engine since the release of Half-Life 2, primarily its high dynamic range rendering capabilities and the upgraded facial animation system.

Episode One was released together with Half-Life Deathmatch: Source, a port of the original Half-Life's multiplayer, which doubles as Episode One's multiplayer component. The retail copies of Episode One also come with Half-Life 2 Deathmatch for those who have not previously purchased the later title.