Day of Defeat: Source is a team-based first-person shooter multiplayer video game developed by Valve Corporation. Set in World War II, the game is an updated version of Day of Defeat, moving from the GoldSrc engine used by its predecessor to the Source engine. The game was released for Microsoft Windows on 26 September 2005, distributed through Valve's online content delivery service Steam. Retail distribution of the game was handled by Electronic Arts.
The game was officially announced in February 2005. During the course of its development, Day of Defeat: Source progressed from being a straight conversion of Day of Defeat, to the alteration of certain aspects of the game's design and introduction of several new features. In addition, Day of Defeat: Source has been used by Valve to present new design features on the Source engine, such as high dynamic range rendering and cinematic effects.