Rosberg's intended partner at Mercedes in 2010 is not yet known
Nico Rosberg will drive for the new Mercedes F1 team in 2010.
The 24-year-old announced his departure from Williams in October and, as a German native, had been expected to drive for Mercedes next season.
The new team has been formed after Mercedes-Benz bought a 75% stake in Brawn GP, the team that won the 2009 drivers' and constructors' titles.
Seven-time world champion Michael Schumacher, 41, remains in the frame to take the second seat at Mercedes.
He would have to be tempted out of retirement to reclaim a spot behind the wheel in F1, while Germany's Nick Heidfeld has also been linked and is seen as a safe pair of hands.
When asked to rule out the possibility of signing Schumacher, Mercedes motorsport vice-president Norbert Haug and Brawn chief executive Nick Fry both failed to do so on Monday.
"We are here today to talk about Nico and I think we should stick to that subject," said Fry.
Haug added: "I fully understand that these speculations are ongoing as long as we do not announce who is driving the other car, but that's what we have to live with at the moment."
Fry said the initial approach to Rosberg had been made during the season and before Mercedes expressed an interest in buying the Brawn team.
"Nico was due to join Brawn under any circumstance," he said.
"We are extremely pleased that the circumstances have worked out as they have done but the negotiation was done principally with the Brawn team and myself and Ross (Brawn)."
Rosberg said: "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 at Bahrain on 14 March."
"No other brand in Formula 1 has such a long and successful tradition.
"I am very proud that I will race for the new team from the start and look forward to working with Ross Brawn."
Team principal Brawn added: "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.
"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."
Rosberg had spent the last four years competing for Williams and is yet to win a race in 70 starts. The son of Finland's 1982 champion Keke, he has a best finish of second in Singapore in 2008.
Mercedes took over British-based Brawn last week and both the team's 2009 drivers have moved to pastures new. World champion Jenson Button has joined McLaren, while Brazilian Rubens Barrichello has taken Rosberg's place at Williams.