Carbisdale Castle, which overlooks the Kyle of Sutherland, was shut down three years ago.
The Scottish Youth Hostels Association (SYHA) had to close it because of rising repair costs.
It has been sold for an undisclosed sum to FCFM Group Ltd which wants to return the castle "to its former glory".
The company said it wanted to quickly complete repair and renovation works started by the SYHA.
SYHA chief executive Keith Legge said: "SYHA is proud to have provided an affordable experience of staying in a Scottish castle for some 65 years, hosting in that time one million guests, including young people and families from all around the world.
"As a self-funding Scottish charity of 85 years, it was with great sadness that SYHA was unable to sustain the castle as a youth hostel.
"All proceeds from the sale of Carbisdale Castle will be reinvested back into modernising the SYHA network of youth hostels across Scotland."
The Dowager Duchess of Sutherland had Carbisdale Castle built between 1907 and 1917 following the death of her husband, George Granville William Sutherland-Leveson-Gower, the Third Duke of Sutherland.
Lady Mary was the duke's second wife and after he died she became embroiled in a legal dispute over his will with her step son, the fourth Duke of Sutherland.
Following the family feud, it was dubbed the Castle of Spite and, according to local legend, is haunted by a female ghost called Betty.
The castle was bought by a Scots-Norwegian family in 1933 and was used as a refuge for the Norwegian royal family during World War II.
One of Scotland's most spectacular youth hostels has been bought by a company planning to make it "a world-class private residence".