The news agency said the man, reportedly Australian, dressed in a bishop's outfit comprised of a shorter-than-usual cassock, an unusual crucifix chain and a purple scarf worn to look like an Episcopal sash to get into the Monday meeting.
The meeting of more than 100 cardinals from around the world precedes the upcoming secret meeting of the conclave to elect a new pope.
The man was eventually discovered and ejected in view of journalists, ANSA said.
Benedict Cumberbatch's dramatic reading of R. Kelly lyrics is just what you need
Couple mistakenly served bag of cash at McDonald's drive-thru