Keselowski, 25, replaces David Stremme in the No. 12 Dodge for next year's Sprint Cup campaign. It wasn't clear what role Stremme will have with the team in 2010.
In a limited Cup schedule this year with Hendrick Motorsports and Phoenix Racing, Keselowski is third in Nationwide points.
"We are very excited to welcome Brad to the Penske Racing family," team owner Roger Penske said. "Brad has shown terrific success on track the past several years and has quickly established himself as one of the top young stars in racing. He comes from an impressive family background in motorsports. He is a proven winner and a welcome addition to our team."
Keselowski has one win and three Top 10 finishes in 10 career Cup starts.
"This is a great opportunity for me to continue competing in the Nationwide Series and to run my first full season in the Cup Series with a strong and experienced team," said Keselowski, a third generation driver.