Christie's said the painting has not been seen in public for almost 40 years, and has not been offered at auction since 1930.
Christie's Old Masters expert Richard Knight said the 1658 painting - which depicts an unknown subject, hands on hips in a defiant pose - was "a truly remarkable portrait" from one of Rembrandt's most artistically fertile periods.
The painting was donated to New York's Columbia University in 1958 by George Huntington Hartford II, the art-loving heir to the A&P supermarket chain.
When left-wing students occupied the university president's office in 1968, authorities - terrified for the painting's safety - sent in security officials to remove it and put it into storage.
Christie's said the painting has not been on display to the public since it was part of an exhibition at the Detroit Institute of the Arts in 1970. The university sold it privately a few years later.
Christie's did not identify the seller, saying only that the painting was in "a distinguished private collection."