The emotional actress took the stage moments ago at the Kodak Theatre to accept the award from 2010 Best Actor winner Jeff Bridges.
"The prize tonight was to get to work with my fellow nominess," she said in her acceptance, before thanking her parents and her professional team.
Portman, who is pregnant by and engaged to her "Black Swan" co-star Benjamin Millepied, was considered this year's favorite. She wore a purple Rodarte dress and matching Tiffany earrings, in the shape of tassels.
She beat out Annette Bening ("The Kids Are All Right"), Nicole Kidman ("Rabbit Hole"), Michelle Williams ("Blue Valentine") and Jennifer Lawrence ("Winter's Bone").
And to see who else went home with Oscar, see the list of winners at Moviefone.