Found in a private collection in the north-east of Scotland, the letter, unearthed as part of a lottery-funded project to bring the Bard to children, brings to light a friendship between Burns and an actress, Elizabeth Kemble, and mentions a secret cache of letters.
The letter was discovered by Irene Furneaux, a volunteer historian with the Heritage Lottery-funded Enlightened Burns project, who was researching books and archives to find out more about Burns's connections to Freemasonry and slavery when she stumbled across the letter.
The research was being carried out as part of the project through which senior school pupils were putting on a play called Liberty, as part of the Aberdeen Youth Festival.
The letter, which has been authenticated, was written to the Ms Kemble, nee Satchell, in 1794.
In the document, Burns says: "I venture to send you a manuscript of mine which has very little other value than its being a private thing."
He adds: "All I have to ask of you is, lay the book under lock & key, when you go out, as you will easily believe that I do not wish to expose such a thing to the random perusal of Chance."
This document was a collection of his letters which he had collected "as a boon of Friendship to a much valued Character who is, Alas! Now no more".
It is understood this character is Robert Riddell of Glenriddell, a landowner who was a friend of the poet for several years.
Glenriddell was a radical who shared the same political sympathies as Burns, and the Kemble family. The Kembles were not only actors but also owned theatres, including the first theatre in Aberdeen and the Haymarket Theatre in Edinburgh.
The Kembles were Freemasons and great supporters of the abolition of slavery, as was Burns.
Elizabeth Kemble is best known for her performance in Inkle and Yarico, an anti-slavery play which was popular with radicals at that time.
Last night, Professor Alan Riach, an expert in Scottish literature at Glasgow University, said: "I didn't know of this letter though there is no shortage of undiscovered material in the archives of Scottish literature, and far more to be unearthed beyond Burns, great as he is.
"There is a glimpse here I think of the sharp edge of things that were not easily to be debated in public, the sense both of an intimacy of sympathy and a threatening political context. One thinks of Burns often as outspoken, forthright, a powerful presence, but there is also this quality of private understanding, intuitive feeling, common purpose."
Helena Anderson Wright, project director of Enlightened Burns, said: "It is quite remark-able that over 200 years after Burns's death a find like this is still possible.
"Now that we have had it authenticated, we are delighted to share this letter with the world.
"There is still a mystery surrounding its complete interpretation which will no doubt be hotly debated by academics for years to come."
Colin McLean, head of the Heritage Lottery Fund, added: "We knew that through this project hundreds of children and young people would discover the life and works of Burns, but we never for a moment anticipated a discovery such as this.
"Robert Burns is one of the most celebrated figures in Scottish culture - this letter gives us a glimpse into a part of his life, which until now was unknown, as we piece together our understanding of the man."
l The world's most southerly Burns Supper will take place tonight in Antarctica – thanks to a Scottish university graduate and an ancient can of haggis.
Florence Barrow, 22, who graduated from Edinburgh University, has inspired her colleagues at Port Lockroy to mark the Bard's birthday.
She recently took up her position as postmistress, working alongside three other hardy Brits at the remote gift shop and research station.
The 40-year-old can of haggis will form the centrepiece of the celebration along with oatcakes and tablet from the gift shop.