The painting now hanging in the dining room of 16th-century English sea captain Sir Francis Drake's former home, Buckland Abbey in Devon, is 'signed' Rembrandt and dated 1635.
In the 91 x 72 cm portrait, 29-year-old Rembrandt is wearing a black velvet cap with two ostrich feathers, a gorget (a decorative metal band worn round the neck) and a short, decorated velvet cape.
The painting was acquired by the trust in September 2010 as a gift from the estate of the late Edna, Lady Samuel of Wych Cross.
"This portrait is now one of our most important works of art, and will be the only Rembrandt in the National Trust's collection of approximately 13,500 paintings," David Taylor, Curator of Paintings and Sculpture said.
More From This Section
The investigation of the painting led by world's leading Rembrandt expert - Ernst van de Wetering - determined that it was a self-portrait by Rembrandt himself.
"But, over the past 45 years we have gathered far more knowledge about Rembrandt's self-portraits and the fluctuations in his style. In 2005, I published an analysis of the genesis of the painting on the basis of an X-ray," Wetering said in a statement.
"This analysis and newly found circumstantial evidence remarkably increased the likelihood that the painting was by Rembrandt himself.
"My observations of the painting technique during my recent visit to Buckland Abbey were in tune with what I had observed with other paintings of that period among which the Belshazar in the London National Gallery and the (also erroneously rejected) so called Rabbi in the British Royal Collection which show the same crude brushwork and painterly appearance," Wetering said.