The gold coin, pictured here, which bears the king's personal emblem of the boar, is also in pristine condition, say the auctioneers, who will include it in their December 4-5 sale in London.
It dates from c.1484 and has survived untouched by weather or humans over many centuries. While badges, sword mounts and cannonballs, as well as other coins have been found near the site before, this find is very rare.
Spink specialist William MacKay said: "It is incredible that just as we are having all the publicity about the discovery of Richard III's tomb in Leicester, this coin, which was found so close to where he met his death, should be brought into our offices. It's a remarkable find and shows little sign of wear and tear, making this very likely a coin lost at the time of the Battle of Bosworth and Richard III's death."
The coin features St Michael spearing a dragon and on the reverse, a ship on the waves, with a shield and crucifix above it. It is estimated to fetch £12,000-15,000.