Kazuki Nakajima will continue at Williams next year after the Grove-based team confirmed an unchanged driver line-up for 2009.
The Japanese driver has scored several points finishes this year and also has close links with Williams's engine partner Toyota.
Rosberg was already signed up for the 2009 season after extending his contract last winter. Although having a disappointing middle part to this season, the German has taken two podium finishes - including a best-ever second place finish in Singapore last weekend.
The team have high hopes that the two drivers, plus tester Nico Hulkenberg, will help them make further progress up the grid next year.
Team boss Frank Williams said: "I am delighted we are able to maintain continuity with our drivers going into 2009.
"Nico Rosberg continues to be one of the most capable drivers in the sport. Kazuki is getting stronger all the time and has more to offer and Nico Hulkenberg is developing well."