The auction house Christie's says a previously unknown foot sketch by Michelangelo could fetch $1.5 to $2 million (£1.1 to £1.47 million).
The piece is set to be offered on Thursday during a live auction of Old Master Drawings in New York.
Experts at Christie's say it is a foot sketch of the Libyan Sibyl which was later painted on the Sistine Chapel ceiling.
According to the auction house, the unsuspecting owner had submitted a photograph for an auction estimate only to discover the sketch's true value.
Andrew Fletcher, global head of Christie's Old Masters Department, described the discovery as "one of the most memorable moments" of his career.
