Beckett made his postseason debut in the opening game of the 2003 National League (NL) Division Series, where he allowed only one run in seven innings against the San Francisco Giants but picked up the loss as Jason Schmidt pitched a complete game shutout. However, the Marlins came back to win the series in four games. His next start, in game one of the 2003 NL Championship Series against the Chicago Cubs, did not go well as he allowed four runs in the first inning, two of them on a home run by Moisés Alou, and six runs in his innings. The Marlins battled back and won the game in extra innings. The Cubs took the next three games and only needed one more win to take the series when Beckett returned to the mound in game five. He allowed only three Cubs to reach base in the game, on two singles and a walk, in the complete game shutout to keep his team alive. The Marlins took game six also, thanks partly to the infamous Steve Bartman incident. In game seven, on two days rest, Beckett came in out of the bullpen to pitch four innings and the Marlins won 9–6 to clinch the National League championship.