MotoGP world champion Valentino Rossi has signed a two-year deal with Yamaha.
Rossi clinched three titles in a row with Honda but announced that he would be leaving them after winning the last race of the season in Valencia.
The Italian, who will team up with Spaniard Carlos Checa at Yamaha, will start testing when his current contract with Honda ends on 31 December.
Yamaha last had a world champion in 1992 when American Wayne Rainey won the third of his 500cc titles.
"We are extremely happy to welcome Rossi into the Yamaha team for the 2004 season," said Yamaha Motor Racing director Lin Jarvis.
"During our discussions with Valentino, it was immediately clear that he was as excited as us about our shared future challenge.
"His incredible talent will be a great asset. We obviously have an enormous amount of work ahead of us, but we are confident that we can give Valentino the means to challenge for the title once again.
"It is extremely important for Yamaha to return to the very highest level of Grand Prix."
Rossi, 24, is one of only three riders to have won the world title in the 125cc, 250cc and 500cc/MotoGP categories.
He cruised to his third title in the premier class this year after taking victory in nine of the 16 races.
His decision to leave behind the dominant Honda RCV bike is a gamble, as it has brought him 20 wins in the last 32 races.
The top three riders in this year's championship all rode versions of the four-stroke machine.
But Rossi said he wanted to make the move from Honda because he no longer found winning on the RCV a challenge.
"To make this choice is a little bit crazy, but we'll find out next year how crazy," said Rossi.
Since his new team's main sponsors are Gauloises and Fortuna cigarettes, it seems likely that Rossi will race with tobacco sponsorship for the first time, something he previously said he would not do.
His new team-mate Checa was seventh in the championship last season with a best finish of fourth.
Rossi's arrival means Italy's 2002 250cc champion Marco Melandri will move to the Tech 3 Yamaha team.