The Vatican says Jozef Wesolowski has placed an appeal on the punishment and it will be heard "without delay".
Spokesman Fr Federico Lombardi said: "Former nuncio Josef Wesolowski has recently appealed, within the prescribed limit of two months, against the most serious canonical sentence, that of a return to the lay state, which has been imposed upon him".
Fr Wesolowski, from Poland, was expelled from the priest hood in late June amid allegations of a sex assault on a child in the Dominican Republic.
He was an archbishop in the Caribbean nation but now faces criminal proceedings by the Vatican.
If he's found guilty he could be extradited back to the Dominican Republic to face another trial.
Fr Lombardi said Pope Francis wanted the case looked at "justly and rigorously" and was being "duly and carefully informed".