Anthony Zonfrell, 20, was visiting the Vatican City when he saw a portrait of Pope Gregory IX in a fresco by Raphael. The figure in question is to the far left of the painting.
"It looked just like Sylvester Stallone," he said.
After he uploaded a photograph of the image to Reddit, it quickly received over 700,000 views.
The painting forms part of Raphael's decoration of the Apostolic Palace in the Vatican City, which became known as the 'Stanze di Raffaello', or the Raphael Rooms.
The Cardinal and Theological Virtues, which contains the 'Stallone' image, was painted in 1511.
It is believed that Raphael originally sketched the outline of the fresco, while Lorenzo Lotto completed the painting.
Stallone is currently completing his upcoming film Bullet in the Head, to be released in February 2013.
He will next appear alongside Arnold Schwarzenegger and Bruce Willis in The Expendables 2, which is released on August 17.
Watch Sylvester Stallone in the Expendables 2 trailer below: