An actor known for his role as an Italian Mafia assassin is now promoting and exhibiting an old master painting of a bloodied, dying saint.

Federico Castelluccio, who played the ponytailed hired gun Furio Giunta on “The Sopranos” TV series, has rediscovered the artwork, a depiction of St. Sebastian said to have been painted by Guercino in the 1630s. This week, in its first American showing, the painting went on view at the Princeton University Art Museum after its European debut in a survey of St. Sebastian portraits at a castle near Turin, Italy, last year.

Guercino experts including David Stone, a professor at the University of Delaware, and Nicholas Turner, an independent art historian, have authenticated the piece, which Mr. Castelluccio said he found five years ago at a German auction house; it was labeled an anonymous 18th-century work. It shows St. Sebastian nearly naked and gazing heavenward as blood drips from arrow wounds in his torso.

Mr. Castelluccio and an unnamed co-investor bought it for about $70,000, and its value has been estimated at up to $10 million. He and the co-owner may eventually sell the portrait, he said, ideally to a prestigious institution.