The 2003 V8 Supercar Championship Series was a motor racing series for V8 Supercars. The series, which was the fifth V8 Supercar Championship Series, began on 22 March 2003 in Adelaide and ended on 30 November at Eastern Creek Raceway after 13 rounds. It ended with the awarding of the 44th Australian Touring Car Championship title by the Confederation of Australian Motor Sport to Tasmanian driver Marcos Ambrose. It was the first time a Stone Brothers Racing driver had won the championship and marked the first title win by a Ford driver since Glenn Seton in 1997, ending a five-year run by Holden Racing Team drivers.

The Supercars Championship currently known as the Repco Supercars Championship under sponsorship, is a touring car racing category in Australia and New Zealand, running as an International Series under Fédération Internationale de l'Automobile (FIA) regulations, governing the sport.