Judge Gianluigi Marrone, who is a member of the court system of the independent Vatican city-state, said the man was wearing clerical garb and carried documents alleging that he was a priest.
"Some time ago I had to deal with an unusual case - a fake priest," Marrone told the Vatican newspaper L'Osservatore Romano.
"He was caught by surprise in the basilica while he was trying to take his place in a confessional.
"He was wearing clerical garb, but the expert eye of our (basilica) personnel didn't need much to sense something strange in his behavior," Marrone said without elaborating.
The man was stopped and his documents checked, and even though he had what appeared to be legitimate documents, including a Vatican pass, personnel were still suspicious, the judge recalled.
Checking with Italian authorities about the documents "unmasked him," Marrone said, adding that the man had passed himself off as a priest in Italy.
"It was a case of usurping an ecclesiastical title, and thus he was tried by our tribunal," the judge added.
Marrone didn't say when the incident happened, what the tribunal's verdict was or if the man received any punishment.
Vatican judicial offices were closed Saturday afternoon, and no-one answered the phone at the judge's home in Rome.
Marrone said much of the tribunal's workload involves cases of people having their wallets stolen in the basilica or in the Vatican museums, with most of those crimes going unsolved.
If the culprits are found, they're turned over to Italian police, he said.
Last year, Italian news reports said that for the first time the Vatican court system issued a drug conviction, giving a former employee of the Holy See a four-month suspended sentence for possession of cocaine.
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