A publicist for Portman confirmed Monday that the couple are engaged and expecting, but declined to give any further details. People magazine first reported the news.
The 29-year-old actress and Millepied, a well-regarded ballet dancer and choreographer, met during the making of "Black Swan," Darren Aronofsky's psychological thriller that stars Portman as a ballet dancer. She's been nominated for best actress by the Golden Globes and the Screen Actors Guild.
Millepied played a small on-screen role in the film as a dancer.
Portman also stars in Ivan Reitman's upcoming romantic comedy, "No Strings Attached."