The photographic image of Queen Elizabeth II is by Julian Calder and is titled Queen of Scots, Sovereign of The Most Ancient and Most Noble Order of The Thistle and Chief of The Chiefs. It shows her standing by the Gelder Burn on the Balmoral estate in Royal Deeside.
The portrait will hang in the Scottish National Portrait Gallery in Edinburgh.
The work was acquired with the aid of the Patrons of the National Galleries of Scotland and the NGS said it would not disclose the cost of the purchase.
Christopher Baker, director of the Scottish National Portrait Gallery, said: "This unusual and impressive portrait of Her Majesty The Queen is a very welcome addition to the Scottish National Portrait Gallery's collection. Her Majesty visited the Gallery in 2012 to formally re-open, following its highly successful re-development, and so it is especially pleasing to be able to show here such a striking and distinctly Scottish portrait, which represents an accomplished and fresh interpretation of traditional imagery."