Act III takes back in the chapel of Act I. Gris lidis sits alone in the chapel. She wants to pray to St. Agnes and open the alterpiece and finds that St. Agnes is gone. The Devil enters as a messenger from a pirate who will return her son for a kiss. He reminds her of his faithlessness and gets her to follow him. As they are about to leave the chapel, she takes some holy water and sprinkles it over the Devil.
Sparks begin to fly and they exit.
The Marquis returns wounded from the Crusade. The disguised Devil relates the story of Gris lidis' infidelity. The Devil gives him a knife to kill his wife. Just as he is about to go off, he recognizes the ring on the Devil's finger and its wearer. He welcomes her as she re-enters the chapel.