The 2003–04 UEFA Champions League was the 12th season of UEFA's premier European club football tournament, the UEFA Champions League, since its rebranding from the European Cup in 1992, and the 49th tournament overall. The competition was won by Portugal's Porto, who defeated AS Monaco of France 3–0 at the Arena AufSchalke in Gelsenkirchen, Germany for Portugal's first win since 1987. This was Porto's second European trophy in two years, following their UEFA Cup success from the previous season. This was the first UEFA Champions League competition to feature a 16-team knockout round instead of a second group stage.
Location of teams of the 2003–04 UEFA Champions League group stage. Brown: Group A; Red: Group B; Orange: Group C; Yellow: Group D; Green: Group E; Blue: Group F; Purple: Group G; Pink: Group H.
16 winners from the third qualifying round, 10 champions from countries ranked 1–10, and six second-placed teams from countries ranked 1–6 were drawn into eight groups of four teams each. The top two teams in each group advanced to the Champions League play-offs, while the third-placed teams advanced to the Third Round of the UEFA Cup.
Tiebreakers, if necessary, were applied in the following order:
Points earned in head-to-head matches between the tied teams.
Total goals scored in head-to-head matches between the tied teams.
Away goals scored in head-to-head matches between the tied teams.