ROME—A Vatican court on Saturday found Italian Cardinal Giovanni Angelo Becciu guilty of embezzlement and fraud and sentenced him to 5½ years in prison, marking a stunning fall from grace for a man who was once one of the most powerful advisers to the pope.

Becciu, who has denied any wrongdoing, was the first cardinal to be tried in Vatican City’s criminal court. “We reiterate the innocence of Cardinal Angelo Becciu and we will appeal,” his lawyer, Fabio Viglione, told reporters after the verdict and sentence were announced.