Made during the reign of King David I of Scotland, the penny was unearthed by a metal detector in a field in the north of England.
Experts think it could fetch between Â£8,000 and Â£12,000 when it goes up for auction on 22 September.
The penny bears the head of King David I, who ruled Scotland from 1124 to 1153.
It is thought to have been struck some time after the Scots invaded England in 1136 to intervene in a civil war over the English throne.
The coin will be auctioned in London by international coin specialists Dix Noonan Webb, and the auctioneers have described it as very important.
It is one of the first to be struck bearing the head of a Scottish monarch and represents a key moment in the development of independent Scotland.
The 54-year-old metal detectorist who found it has asked not to be named.
He discovered the coin in a field in County Durham, on the site of a long-lost medieval village after he was given permission to search the area two years ago.
He also turned up pennies from the reigns of English kings Edward I and Edward III as well as Elizabethan coins, two modern gold rings and a large number of bullets.
The coin was almost certainly dropped in the medieval village which, in the 12th Century, was in an area of Northumbria granted to David I under the Treaty of Durham in 1139.
It may, therefore, have lain in the soil for almost 900 years.
Hiram Brown, of the Edinburgh Coin Shop, told BBC Scotland: "This coin is unique because it has an Edinburgh side with David on it, and the other side is Carlisle. It was probably struck in Carlisle.
"David and his son Prince Henry attacked Carlisle and took over silver mines in that area and struck coins.
"They went on to strike coins in many places in Scotland."

Daniel Swift sent explicit pictures to the 14-year-old and arranged for her to travel to Lincoln to meet him for sex.
He was caught when those behind the sting detained him and called police.
Swift, 47, of Princess Street, admitted charges of attempting to sexually communicate with a child and attempting to meet a child following grooming.
He was ordered to be detained in a secure psychiatric unit after Lincoln Crown Court heard he had been treated for over 25 years for mental health problems and had been diagnosed as suffering from paranoid schizophrenia.
Richard Thatcher, prosecuting, told the court: "This case arises through the defendant's contact through social media with, as he believed her to be, a 14-year-old girl called Emma.
"She was actually an adult woman who had set up that alter ego on social media," he said.
The court heard he first made contact with 'Emma' in April when she told him her age.
"He began by asking if she wanted a daddy to look after her," Mr Thatcher told the court.
"There was a conversation as to whether this involved him becoming involved with her mother. He told her 'Daddy loves his little girls not their mummies'."
The court heard Swift went on to send an image of his penis as well as explicit videos.
He made it clear he wanted to have sex and at one point he suggested a foursome with Emma, her 10-year-old sister and 14-year-old cousin.
Judge Simon Hirst also imposed a sexual harm prevention order and placed Swift on the sexual offences register for five years.

The 28-year-old Belgian was injured when he landed awkwardly after he appeared to be elbowed by Palace striker Connor Wickham.
Wickham has been charged with violent conduct by the Football Association.
Tottenham said: "Our medical staff will continue to monitor his progress during his rehabilitation to determine when he will be ready to return to training."
Chelsea goalkeeper Thibaut Courtois was out for three months earlier this season with a medial ligament injury.
One of the earliest coins ever minted in Scotland is going under the hammer.
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A man who groomed and arranged to meet what he believed to be a young school girl was trapped by internet vigilantes, a court has heard.
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