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."
One of the earliest coins ever minted in Scotland is going under the hammer.