Unreal Tournament 2003 or UT2003 is a first-person shooter video game designed mainly for multiplayer gaming although the game had a built in single-player mode that mimics multiplayer gaming by featuring AI-bots. The game is part of the Unreal franchise's series of games and is a sequel to 1999's Unreal Tournament. Unreal II: The Awakening was released as a sister product to the game, however, was developed for single-player only. Later, Unreal II would receive an add-on that would enable a multiplayer mode. UT2003 was followed by Unreal Tournament 2004 released in March of 2004.

UT2004 completely replaced UT2003 on store shelves and UT2004 boxes sold in the United States include a voucher that along with proof of having bought UT2003 gives a discount for UT2004.

UT2003 set a record for the number of downloads (1.2 Million) when the demo was released, which is a reflection of the popularity of the original UT. In addition, the game engine has been widely licensed for games such as Tom Clancy's Rainbow Six, Splinter Cell, and America's Army.