The 25-year-old is currently third in the championship standings but had already been told he would not be kept on by his Repsol Honda team.
The Italian will partner Cal Crutchlow next season with fellow Brit Bradley Smith set to join the team in 2013.
Dovizioso said: "I am excited to become part of the squad and to be teamed with Cal. This is a new challenge."
He added: "Now that 2012 is sorted, I want to focus on finishing the season in the best possible way."
Dovizioso will replace Colin Edwards and his signing leaves Crutchlow's longer-term future in MotoGP unclear.
The Coventry-born rider is contracted for 2012 but with Smith tied up for 2013, only one spot in the team will be available after next season.
Tech 3 team manager Herve Poncharal said: "Andrea 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 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."
