At the close of the 2003 season, Valentino Rossi and Honda's racing division, HRC, had parted ways. The departure was acrimonious enough that HRC held Rossi to the strict terms of his contract, which prevented him from riding any other manufacturer's machine until 31 December 2003. Rossi's move to Yamaha was widely considered a significant gamble: the manufacturer had not won a premier-class world championship in 12 years, and the YZR-M1 had shown flashes of potential but had not matched the dominant Honda RC211V.
The season also saw d'Antin Team switch from Yamaha to Ducati after a five-year partnership, marking the first time Ducati had introduced a satellite customer team in MotoGP.
Rossi won on his Yamaha debut in South Africa and did not look back. By winning the 2004 title, he achieved something no rider had managed since Eddie Lawson: back-to-back premier-class championships on different manufacturers, having won with Honda in 2003 and Yamaha in 2004. The achievement silenced debate about whether Rossi's success owed more to his machinery than his talent.
Sete Gibernau on Honda provided the most sustained challenge early in the season, but faded as the year progressed. The relationship between Gibernau and Rossi, once collegial, deteriorated significantly during the campaign, with the Qatar round a particular flashpoint in their rivalry.
A notable incident occurred at the Italian Grand Prix at Mugello. After 17 laps, rain intervened and the race was restarted over 6 laps under the rules then in force, with the initial 17 laps counting only for grid positions on the restart rather than contributing to the race result. Conditions had improved enough that riders started the second part on slick tyres. The regulations were revised ahead of 2005 to prevent races from being stopped and restarted in this manner.
Rubén Xaus was awarded the Rookie of the Year honour in the MotoGP class. A new round was added to the calendar: the Qatar Grand Prix, run on a Saturday, became the first motorcycle Grand Prix held under floodlights. The Japanese Grand Prix also relocated permanently from Suzuka to the Twin Ring Motegi following the fatal accident of Daijiro Kato at Suzuka in 2003.
The 250cc world championship was won by Daniel Pedrosa, riding for Honda. Pedrosa, a teenager who had shown exceptional pace in the 125cc class, converted his talent into a dominant 250cc campaign and confirmed himself as a future premier-class contender.
Andrea Dovizioso claimed the 125cc world championship on a Honda. Dovizioso's title was the beginning of a long career in Grand Prix motorcycle racing that would later see him become a MotoGP race winner and championship challenger across more than a decade in the premier class.
The 2004 season reinforced the primacy of rider talent at the highest level of Grand Prix motorcycle racing. Rossi's ability to walk into a manufacturer widely regarded as the underdog and immediately deliver a championship was without precedent in the modern era. His first season with Yamaha transformed the competitive landscape: rivals could no longer attribute Honda's dominance solely to the RC211V, and the sport's manufacturers entered an arms race to attract and retain the best riding talent alongside their engineering investment. The season is frequently cited as the moment when Rossi's status as the greatest of his generation was cemented beyond serious dispute.