Dressed in a priest's clerical clothing, the man accessed staff-only areas of the famous Duomo, separate from the areas open to tourists or the congregation.
He was even pictured speaking to nuns in the cathedral during his visit.
When he was spotted by police officers, they asked him to show his ID, at which point the fake priest at first tried to run away.
Then he offered a cover story, allegedly claiming to be an Orthodox priest - but police quickly saw through this, partly due to the fact he was wearing the wrong kind of garments.
According to Milan-based Il Giorno, the man was from Monza and explained that he thought up the plan in order to avoid paying for the €3 entry ticket.
However, he said he had bought the priest's clothes shortly beforehand for €70 - clothes which were then confiscated by police.
Photo: Miguel Medina/AFP
Police officers escorted him out of the cathedral, accompanied by soldiers who are on duty at the site as part of Italy's anti-terror measures which have seen security ramped up at high-profile monuments.