Nico Rosberg has been confirmed as the first driver for the Mercedes GP team that world champion squad Brawn will become in 2010.
The German had long been the hot favourite for a seat in a Mercedes-powered car next year even before the company announced its takeover of Brawn, and said he was thrilled to be part of the project.
"I am really happy to be a part of the Silver Arrows re-launch in 2010 as a driver for Mercedes," said Rosberg.
"No other brand in Formula 1 can look back on such a long and successful tradition in motor racing.
"I am very proud that I will now drive for the new Mercedes team and work with Ross Brawn.
"I am more motivated than ever and can hardly wait to start testing with the new Silver Arrow and for the first race of the new season."
Rosberg has spent his whole Formula 1 career so far with the Williams team.
He was promoted from testing duties in 2006, having won the inaugural GP2 title in the previous year, and stunned F1 by charging from last to seventh and setting the fastest lap in his debut race following a first corner tangle.
But with Williams not a consistent front-runner in recent years, Rosberg is still hunting for his first grand prix win.
He took two podium finishes last season, and although he could not repeat those results this year, a more consistent points-scoring season gave him a career-best seventh place in the drivers' championship.
Mercedes chief Norbert Haug said he had long been a fan of Rosberg.
"I saw Nico racing karts alongside Lewis Hamilton and later in the support programme of the DTM events - I have known him since he was a young boy," Haug said.
"Early on it was obvious that he would make his way as a race driver and we are glad that after four years with Williams he will now be one of our drivers at Mercedes."
Brawn, who is staying on as team boss in the Mercedes era, said Rosberg's experience would be valuable.
"Nico is a great talent, and with four years of experience in Formula 1, is a driver who will be able to make a valuable contribution to our team right from the outset," said Brawn.
"I had the pleasure of working with his father Keke during his Formula 1 career and it is great to see Nico following in his footsteps.
"2009 was Nico's best season in Formula 1 to date and we look forward to seeing his development continue with us at Mercedes next year."
Mercedes has yet to announce its second driver, but with rumours that it was trying to lure Michael Schumacher into a comeback having been played down, ex-BMW driver Nick Heidfeld remains the favourite for the seat.