Brazilian Rubens Barrichello is to leave Ferrari at the end of this season to join BAR-Honda.
The 33-year-old, who has won nine Grands Prix for Ferrari, has signed what BAR said was a "multi-year deal".
He will end his contract with Ferrari a year early after getting fed up playing second fiddle to team leader and seven-time champion Michael Schumacher.
"I am very excited to confirm I will be at BAR. I wish to thank Ferrari for six fantastic years," said Barrichello.
"I have won many races with them and I am proud to have helped the team, to date, to win five constructors' world championships.
"However, my target remains winning the drivers' title and to achieve that I felt that I needed a renewed motivation."
Barrichello said he had been impressed by BAR's progress in recent years.
"I have been struck by the spirit and determination of a team that has all the right ingredients to win races and go on to win titles," he said.
"I can't think of anything more exciting than working with the team and Honda to achieve our shared ambitions together.
"Obviously I have a job to do at Ferrari until the end of the season, but I am very happy about the fantastic times that I know still lie ahead."
BAR chief executive Nick Fry called the signing "another exciting milestone" for his team.
"Rubens is not only an excellent driver, but he has the experience of building a race and championship winning team.
"I would also like to thank Ferrari for all of their co-operation in allowing Rubens to join us next year."
Barrichello will have equal status with Jenson Button at BAR if the Briton - who is trying to negotiate his way out of a move to Williams - is allowed to stay at the team.
If Button leaves, the Brazilian will lead BAR.
Barrichello's place as Schumacher's number two at Ferrari will be taken by Felipe Massa, currently with Sauber.