The signature is original and the wooden panel dates from the correct period, the researchers said.
“Materials analysis, stylistic and thematic similarities, alterations made by Rembrandt, and the overall quality of the painting all support the conclusion that this painting is a genuine work,” the gallery said.
The painting has all the hallmarks of Rembrandt at the “peak” of the early part of his career, Dibbits said.
“It’s very high quality. Sometimes with Rembrandt’s portraits you feel that he’s producing in quantity, but with this painting you really feel that he dedicated his soul to it.”
The museum will put the painting on public view from Wednesday.
Its value is not known, but the world record auction price for a Rembrandt painting is £20m, set in 2009.
Other Rembrandt paintings to be sold in recent years include one for £8.6m in 2019, a self-portrait for £12.6m in 2020, and another once-lost Rembrandt work for £11m in 2023.
In 2015, a Rembrandt painting was given a price tag of £35m by the UK government after being sold privately.
Last month, a drawing of a lion by the artist sold for $18m (£13m).