The 2003 American Le Mans Series season was the fifth season for the IMSA American Le Mans Series, and the 33rd overall season of the IMSA GT Championship. It was a series for Le Mans Prototypes (LMP) and Grand Touring (GT) race cars divided into 4 classes: LMP900, LMP675, GTS, and GT. It began March 15, 2003 and ended October 18, 2003 after 9 races. Dyson Racing's overall win at Sonoma Raceway was the first time in ALMS history that the overall win was captured by a vehicle not from the LMP900 class.