JD Fergusson's portrait of a mystery French woman emerged after the National Galleries announced last year they would mount a major retrospective of the key figure in the Colourist movement.
The show's curator, Alice Strang, appealed for any works by the artist, who died in 1961, or his former friends and associates, to come forward.
The owner of the painting, Portrait de Mademoiselle H, contacted the gallery after the appeal appeared in The Herald in December last year. A delighted Ms Strang described receiving "a phone call that curators can only dream of".
JD Fergusson's 1907 work was painted during his influential and successful Paris period, although the identity of his subject is not known.
It was bought by the current owner, who wishes to remain anonymous, in the early 1960s and has not been seen in public since.
It will be included in the Fergusson show at the gallery, which opens on December 7.
Ms Strang said: "I had gone to work during Christmas and New Year to tidy up my desk, when I received the most marvellous phone call. The caller had learned in The Herald that I was trying to find as many works by Fergusson as possible, for possible inclusion in our retrospective.
"He asked if I might like see a painting hanging on his sitting-room wall.
"By that afternoon, I was standing in front of the beautiful Portrait de Mademoiselle H and on the spot I asked if he might be willing to lend it to us."
She added: "It was the kind of phone call that curators can only dream of, and when I saw it I could see how absolutely beautiful it is. Who can say whether this lady was a lover of Fergusson in Paris - we would love to know."
The Edinburgh-based owner said: "I went with my mother to the posthumous sale in about 1962 of Fergusson's works that had remained in his possession - I think at Aitken Dott's [art dealer] in George Street.
"There were two portraits side by side, one being the portrait of Mademoiselle H and the other of a man. My mother said she would buy Mademoiselle H and suggested that I should buy the other one. To my regret I didn't buy it, but she bought Mademoiselle H.
"It hung in her sitting room till her death and since then has been in our sitting room.
"It's a particularly vibrant picture which lights any room in which it hangs and we love it."
Ms Strang has spoken to more than 30 former friends, peers and colleagues of Fergusson for the show, which is the first retrospective of his work for many years.
The Leith-born painter moved to Paris with some of an inheritance left by his father.
Fergusson, met, socialised with and digested the latest advances in French painting during this time, meeting and being influenced by artists such as Derain, Matisse and Picasso.
He met Margaret Morris (who lived from 1891 to 1980) in 1913, and they became a life-long couple.
The First World War forced the couple to return to Britain, and by 1918 Fergusson was a member of the art scene in Chelsea, London, while Morris established the Margaret Morris Club, an important gathering place for local artists, writers and composers.
In 1929, the couple returned to Paris for 11 years before moving to Glasgow, where they settled and were known as "Fergus and Meg".
The couple played a crucial role in Glasgow's art scene, being founder members of the New Art Club - a meeting and exhibiting society - as well as the New Scottish Group.
Fergusson's interest in "Celticism" and the modernist movement also made him a key figure in the Scottish Renaissance movement.
Throughout the 1950s, the couple spent a long period in the south of France, where Morris's dance movements and the students at her Summer Schools became some of Fergusson's favourite subjects.
The Scottish Colourist group, among whom Fergusson achieved the greatest longevity, also included the artists FCB Cadell, Samuel Peploe and GL Hunter.