Federico Castelluccio — the “Sopranos” star who played Sicilian hitman Furio, and is a serious art collector off-screen — has discovered a lost 17th-century painting by artist Guercino that’s worth millions.

“Sopranos” star Federico Castelluccio found this Guercino painting at a Frankfurt art dealer.

Castelluccio, a painter and expert in Baroque European art, found the work at a Frankfurt dealer, but no one knew who the artist was. It was mistakenly marked as an “18th-century Italian holy painting of Saint Sebastian,” the actor told us. “They had no idea who painted this.”

After winning the work at auction, he had it authenticated by experts, who concluded it was a Guercino, circa 1632-34, by using “infrared reflectography,” X-rays and chemical testing of the pigment.

He then also took years to restore it, and it has been seen by experts from the Met.

Estimates are that Castelluccio spent about $140,000 on the work, including the restoration effort, and the painting could now be worth millions.

The actor has created a book proposal about the painting’s incredible journey that’s out to publishers. He also writes about how he became an art lover growing up in New Jersey.

The image will go on display after not being seen publicly for 350 years in a show by the Cosso Foundation and the Miradolo Castle museum in Turin, called “Saint Sebastian: Beauty and Integrity in Art Between the 15th and the 17th Centuries,” that’s opening on Oct. 4.