Auction house Sotheby's said the price was the highest ever paid for a self-portrait of the Dutch master at auction.
Dating from 1634 when the 28-year-old Rembrandt was at the height of his fame and living in luxury in Amsterdam with his new wife Saskia, the picture is the first of his self-portraits to go into auction in three decades.
It was bought by United States casino tycoon Steve Wynn in a transatlantic telephone bid, a Sotheby's spokeswoman said.
"The painting will be put on display at the Wynn Resorts Collection in Las Vegas," she said.
Rembrandt, who towards the end of his life had to sell his house and many paintings by other artists to pay off his high-living debts, painted at least 40 self-portraits.
But the newly discovered image was not recorded in the literature of his works because of the overpainting.
The signed painting, which Sotheby's Old Master expert Alex Bell said was evidently overpainted very soon after it was originally completed, shows the young artist staring boldly out from the canvas from under the shadow of his trademark beret.
It was painted over by one of Rembrandt's pupils and transformed into a picture of a flamboyant Russian aristocrat.
"You have to remember that while Rembrandt was an artistic genius, he was also a commercial artist selling his pictures for a living," Bell said. "If a painting did not sell quickly it would be painted over into a more saleable commodity."
Important event for
connoisseurs of art
The painting floated unrecognised around Europe for 300 years until it was acquired by the previous owner's father in the 1960s. Harbouring a suspicion it might be a Rembrandt, he had some of the overpainting removed but did not finish the job.
It was not until 1999 that the picture, by then clearly identified as an overpainting of a much finer work, was shipped back for detailed investigation to Amsterdam's famed Rijksmuseum, home of Rembrandt's masterpiece "Night Watch."
Using x-rays, infra-red and infinite care, the museum's experts gradually revealed the work with the paint almost as new because of the centuries protected by the layers of overpaint.