The item was found by a couple, who have not been identified, while using a metal detector near Shrewsbury.
It is expected to be snapped up for between £800 to £1,200 in the online sale on Thursday organised by Special Auction Services based in Dudley.
The button dating to1789 is of circular design with a pressed crown image which can be clearly seen above the motto ‘Long Live the King’ on an ornate ribbon, with roses and thistles at the base surrounded by a stipple bezel marking.
It measures 3.5cm long and weighs 7.1 grams and was made in Birmingham to celebrate George III’s recovery from porphyria, a condition which affects the nervous system.
TV auctioneer and expert Thomas Forrester, of Special Auction Service, said: "It is quite a rare find and it was found in the Shrewsbury area by a metal detectorist and his partner. As a youngster he used to accompany his grandad who was an enthusiast.
"George III was said to have this disease which was known to change the colour of his urine and the story was told in the film The Madness of King George. He became miraculously better.
"There was a huge outpouring of support for the king. After this, commemorative goods were made to celebrate his recovery and became a popular practice which was adopted by the Americans following the War of Independence.
"Not many of the buttons were made."
George was King of Great Britain and Ireland from October 25, 1760, until the union of the two kingdoms on January 1, 1801. He then became King of the United Kingdom of Great Britain and Ireland until his death in 1820.
More details are available at specialauctionservices.com