The devil was hidden in the details of clouds at the top of fresco number 20 in the cycle of the scenes in the life and death of St Francis painted by Giotto in the 13th century.
The discovery was made by Italian art historian Chiara Frugone. It shows a profile of a figure with a hooked nose, a sly smile, and dark horns hidden among the clouds in the panel of the scene depicting the death of St Francis.
The figure is difficult to see from the floor of the basilica but emerges clearly in close-up photography.
Sergio Fusetti, the chief restorer of the basilica, said Giotto probably never wanted the image of the devil to be a main part of the fresco and may have painted it in among the clouds "to have a bit of fun."
The master may have painted it to spite someone he knew by portraying him as a devil in the painting, Fusetti said on the convent's website.
The artwork in the basilica in the convent where St Francis is buried was last restored after it was severely damaged by an earthquake in 1997.
Devil found in detail of Giotto fresco in Italy's Assisi
Williams set for Glovers return
Midfielder Andy Williams could make his long-awaited return to first-team action when Yeovil travel to Chesterfield.
Williams has not featured for the Glovers since August 27 due to an ankle injury, but is in line for a place on the substitutes' bench against the Spireites.
Utility player Max Ehmer returns from a one-match ban for Terry Skiverton's men, but Gavin Williams (knee), Jon Obika (knee), Paul Huntington (dislocated finger) and Steve Maclean are all sidelined.
Full-back Luke Ayling has been handed a further one-match suspension by an FA disciplinary panel following his red card against Leyton Orient and will miss the fixture.
Yeovil are rock bottom of npower League One, and go to the b2net Stadium desperately looking to end a nine-match winless run in all competitions.