Former The Sopranos actor Federico Castelluccio has made a huge art find after discovering a painting he picked up at an auction is a forgotten masterpiece worth millions.

The amateur painter and art lover was convinced the piece he picked up at a sale was incorrectly identified, and he called in the experts to make sure.

They confirmed the artwork is in fact a 1630s piece by Italian master Guercino, aka Giovanni Francesco Barbieri.

Castelluccio, who played hitman Furio in The Sopranos, tells the New York Post, "They (auctioneers) had no idea who painted this."

The actor's prized possession will be displayed for the first time in more than 350 years when it makes up part of the upcoming Saint Sebastian: Beauty and Integrity in Art Between the 15th and the 17th Centuries exhibition in Turin, Italy.