The film will be the young star's first full length feature.
She'll play Oscar nominee Jennifer Lawrence's little sister in the movie and director Gary Ross is thrilled the have her onboard.
He says, "It's so exciting to find an amazing young actress like the one we just met. Prim is an emotionally demanding role and in many ways she is the cornerstone of the story. Willow Shields will make an amazing Prim and we're very lucky to have her."