The British actors will star in a three-date run of the musical at the Hollywood Bowl next month.
True Blood actor Moyer will star as lawyer Billy Fynn, while the Les Misérables actress will play murderess Velma Kelly.
Samantha Barks tweeted: "Very excited to announce that I will be playing Velma Kelly in Chicago at this year's Hollywood Bowl!! I'm so excited!"
Meanwhile, Drew Carey will play husband Amos Hart, while Xena: Warrior Princess star Lucy Lawless will appear as prison warden Mama Morton.
The role of Roxie Hart will be announced at a later date.
Chicago will be performed from July 26-28, and is to be directed by actress Brooke Shields, who has starred in both Broadway and West End versions of the play.