The star attended the California ceremony as a Best Actress nominee for her role in Kathryn Bigelow's movie Zero Dark Thirty, based on the real life hunt for terrorist leader Osama bin Laden, and E! News journalist Marc Malkin cornered her for a chat.
Chastain turned the spotlight on the reporter after learning it was his special day, and began an impromptu version of Marilyn Monroe's seductive Happy Birthday song.
In a post on his Twitter.com page, Malkin writes, ""Jessica Chastain sang happy birthday to me on the red carpet Marilyn Monroe style. OMG (Oh my god)! On camera! It was her singing debut on camera!""
Chastain went on to win the Best Actress trophy, and choked back tears onstage as she accepted her prize.