Andrea Dovizioso will race for the Tech 3 Yamaha squad in MotoGP next year alongside Cal Crutchlow.
Dovizioso has spent the past three years with the factory Honda squad, but will leave the squad at the end of the season as it reduces to a two-bike line-up for Casey Stoner and Dani Pedrosa.
The 25-year-old Italian will replace category veteran Colin Edwards, who announced last month that he will ride for Forward Racing when it steps up from Moto2 next year.
"I'm happy to have reached this agreement with Tech 3 for next year and I want to thank [team boss] Herve Poncharal for the opportunity he is giving me.
"I am excited to become part of Herve's squad and to be teamed with Cal. I am sure this new challenge will be exciting, motivating and fun. Now that 2012 is sorted I want to focus on finishing the season in the best possible way."
Poncharal added: "I am delighted that a rider of Andrea's stature has decided to commit his future to Tech 3 and Yamaha. He is without doubt one of the fastest riders in the world who has proven on a regular basis that he is capable of challenging for the podium in MotoGP.
"I am thrilled that he trusts the Tech 3 Team to take his career to the next level and we can look forward to 2012 with high expectations. I am convinced that with Andrea and Cal we have one of the strongest line-ups on the grid in 2012 and both of them will demonstrate the potential of Yamaha's new 1000cc machine."
Dovizioso, the 125cc world champion in 2004, was a front-runner in 250s before making his MotoGP debut with the JiR Honda squad in 2008.
He has spent the last three years with the factory Honda squad, taking 13 podium finishes during that time and recording his lone victory in the premier class at the 2009 British Grand Prix.