The doppelgÃ¤nger was spotted by Harvard student Anthony Zonfrell, 20, who was on holiday in Italy with his family.
He said: â€œMy cousin had told us about how she had seen it when she went, so we were on the lookout.
“As soon as we saw the painting, we knew it was him. My whole family thought it was hilarious. It could actually be a portrait of Stallone.â€
When Anthony, who is from the US, returned from his holiday he uploaded the photograph to community website Reddit.
It has become a viral hit, having been viewed more than 700,000 times.
Anthony added: â€œI posted it at night, and when I woke up it was on the front page.
“I was shocked at the response it got, it really blew up quite fast. All of the comments were hilarious.â€
The Cardinal and Theological Virtues is a 1511 fresco adorning one of the Raphael Rooms at the Vatican.
It depicts Pope Gregory IX approving the Vatical Decretals with a spitting image of Hollywood hardman Sylvester Stallone, 65, looking on. [More]