Valentino Rossi, defending champion riding for Honda's factory Repsol team, won his third consecutive MotoGP title with nine race victories. The season's most striking demonstration of his talent came at Phillip Island, where he received a ten-second penalty for passing under a yellow flag and still won the race by more than ten seconds. As the year progressed, Rossi became increasingly dissatisfied with Honda Racing Corporation and eventually announced he would leave the manufacturer at season's end. He signed with Yamaha and brought crew chief Jeremy Burgess with him, setting up what would become an equally dominant run for a rival constructor.
The season was overshadowed by tragedy at the very first round. Japanese rider Daijiro Kato, reigning 250cc world champion in his second MotoGP season, lost control of his RC211V on the approach to the Casio Triangle at Suzuka and struck a barrier at high speed. Track paramedics restarted his heart at the scene, but Kato never regained consciousness and died two weeks later. Controversy followed because the race was not red-flagged to allow Kato to be removed from the track with maximum medical care. His death was the first of a premier-class rider since Kevin Wrettom in the 1984 500cc season. Suzuka has since been removed from the MotoGP calendar. Kato's teammate Sete Gibernau inherited his factory-specification RC211V for the remainder of the season.
The 2003 season marked Ducati's return to the premier class, following decades of success in Superbike racing. The Bologna manufacturer entered with the Desmosedici GP3, a machine that immediately impressed with its raw speed. Ducati finished the constructors' championship in second place ahead of Yamaha, a result that signalled what the Italian manufacturer would soon achieve. The Honda marque remained dominant, with Honda machines accounting for 15 of the season's 16 premier-class race victories.
The 2003 field introduced several riders who would go on to shape MotoGP for the following decade. Nicky Hayden joined Repsol Honda as a rookie and won the Rookie of the Year award. Troy Bayliss, Marco Melandri, and Colin Edwards also made their MotoGP debuts that season.
The 2003 season introduced mandatory fuel injection for all MotoGP-class machines, following Yamaha's decision to abandon carburettors after 2002. Detailed new sporting regulations were also introduced governing race neutralisation procedures and safety car deployment, reflecting lessons learned from earlier seasons.
Valentino Rossi won the MotoGP riders' championship. Honda took the constructors' title with Ducati second and Yamaha third. In the 250cc class the season produced closely contested results among several manufacturers. The 125cc class was similarly competitive throughout the calendar.
The 2003 season represents a hinge point in MotoGP history. Rossi's departure from Honda to Yamaha, announced at season's end, proved the most consequential rider move of the era and directly led to Yamaha ending Honda's run of dominance. Ducati's competitive debut announced a long-term force that would eventually win championships. The death of Daijiro Kato prompted reforms in circuit safety infrastructure and renewed debate about race direction procedures during serious accidents.