Portman, 29, met Millepied, the principal dancer of the New York Ballet, while filming "Black Swan," the dance drama that has made her a favorite for an Oscar next year.
The actress's representatives would only confirm that she was pregnant and engaged without disclosing the date of the wedding or where the baby will be born.
Millepied, born in 1977 in the French city of Bordeaux, is an internationally-known choreographer who trained Portman and Mila Kunis for their roles in Black Swan, and danced in it as well.
Portman's next film is a romantic comedy with Ashton Kutcher directed by Ivan Reitman, father of Jason Reitman, the director of "Juno" and "Up in the Air."
