“Head of a Bearded Man” was bequeathed to the Ashmolean in Oxford in 1951 and was originally classified as one of the 17th century painter’s early works.
But experts rejected it as a copy in 1982 and it was put into storage.
The museum, which is running a “Young Rembrandt” exhibition until November 1, called in tree-ring dating expert Peter Klein to determine the date of the wood used on the panel.
The dendrochronologist established it came from the same tree used for Rembrandt’s “Andromeda Chained to the Rocks” and Jan Lievens’ “Portrait of Rembrandt’s Mother”.
Both were painted in about 1630 when they were working in Leiden, in the Netherlands.
“The Ashmolean’s ‘Head of a Bearded Man’ was painted on a panel which came from an oak tree in the Baltic region felled between 1618 and 1628,” said Klein.
“Allowing a minimum of two years for the seasoning of the wood, we can firmly date the portrait to 1620-30.”
Museum curator An Van Camp called the discovery “incredibly exciting” and said the work would now go on display alongside other works from the same period.
Further investigation will be carried out after the show ends to determine whether Rembrandt himself painted it, as touches of paint had been added by an unknown hand.
Another version of the painting, which shows an elder man looking downcast, is in the Museum of Fine Arts in Houston, Texas.
Rembrandt’s work from Leiden included similar head studies.
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