Chastain earned an Oscar nomination for her work in "The Help." Her other 2011 film credits include "The Tree of Life," "Take Shelter" and "The Debt."
E! News said she was in talks to play a "sexy scientist" who competes with Gwyneth Paltrow's character Pepper Potts for the hero's affections in the third installment of the "Iron Man" franchise.
But Chastain said she wouldn't be able to play the role as she'd hoped.
"Sorry to say that Iron Man 3 isn't going to work out," she wrote on her Facebook page Monday. "My schedule is jammed packed and I can't fit anything else in. The press announced my possible attachment far too soon. I know many of you wanted me to be involved, and I'm so sorry to disappoint you. Hopefully there'll be another Marvel film in my future. Shane Black and everyone on the IM3 team are really wonderful. I'm very excited to see the film when it comes out. xx"