The 35-year-old two-time Golden Globe winner stunned in a canary yellow Prada gown, which she complemented with shimmering Tiffany & Co. jewels.
Portman walked the carpet with her husband, Benjamin Millepied. The couple, who share a 5-year-old son, Aleph, were all smiles as they posed for the cameras before the show.
Portman spoke with ET's Kevin Frazier on the red carpet, where she admitted that choosing an awards show gown while pregnant requires a significant change in strategy.
"Luckily, I have a wonderful stylist, Kate Young, who really helps me out," Portman explained, "because yeah, you have to [plan] a little bit differently when you're pregnant."
Portman also opened up about playing Jackie Kennedy -- an American fashion icon herself -- and how the formidable figure's famous accent was actually hard to shake once the cameras stopped rolling.
"It was definitely fun to play the accent," Portman shared. "It was so extravagant, how she spoke. She took her time and she really luxuriated in the way she spoke."
"I always think that I drop [the accent] right away," she continued, "but I definitely got some complaints from friends and family that I was still talking in 'Jackie' voice."
Portman also explained that one of the aspects of playing the former first lady that she enjoyed the most was getting to explore who she was as a real human being, instead of the stoic public figure most people think.
"I was able to imagine separate aspects from her and it was incredible to get to think about someone deeply who I had only thought about as an icon before," Portman recounted. "She was really incredible."
"The thing that was so amazing was, at the moment of such private grief, she was able to think about the public and the image of what the family meant and the legacy of what the family meant to this country," she added. "It was a terrifying moment and she, in her resilience in the face of that, really took our country through the darkness."
Recently, ET sat down with Portman, who got candid about the pressure of playing Kennedy, and about how her children are her "good luck charms in life." Check out the video below to hear more.