The "Jumper" co-stars started dating in 2007, announced their engagement in December 2008, only to break up this past August.
"They're dating again," said People's unnamed source without offering details regarding when they resumed seeing each other.
Bilson, 29, is best known for her work on TV's "The O.C." Her film credits include "The Last Kiss" and "New York, I Love You."
Christensen, also 29, starred in the movie "Shattered Glass" and the latest three "Star Wars" pictures.