The Dutch art dealer Jan Six is a direct descendant of a 17th-century burgher who sat for one of Rembrandt’s most important paintings, “Portrait of Jan Six.” It has remained in his family’s possession for 11 generations. Naturally, he grew up to become an old masters specialist.

Mr. Six now says that he has discovered a new Rembrandt, a portrait of an unidentified young man that he purchased at a Christie’s auction in London in 2016 for 137,000 pounds, or about $185,000. If he is right, “Portrait of a Young Gentleman” would be the first wholly unknown Rembrandt painting to be attributed in 44 years — and worth many millions more.

He made the announcement in the Dutch national newspaper NRC Handelsblad on Tuesday, as he published a book, “Rembrandt’s Portrait of a Young Gentleman,” which details evidence he has assembled to support the attribution.