The brooch was found at Kirby Muxloe Castle, Leicestershire, and is dated to the second half of the 15th Century.
The fortress was being built by nobleman William Hastings when he was executed by Richard just a month before he took the crown.
The heart-shaped brooch, uncovered near the moat, may have been a gift from Hastings to his wife, Katherine.
Bearing the French inscription "honor et joie" (honour and joy), it was unearthed in 2016 by a metal detectorist.
Hastings was a close friend and ally of Edward IV and on the king's death in 1483 had supported Richard of Gloucester in his role as Lord Protector.
But with Richard looking to take the crown, and Hastings a champion of Edward's son's claim to the throne, Richard had him arrested and just hours later, executed.
While it has no names on it, the brooch would have only been worn by a high-status woman.
Charles Hanson, of Hanson's Auctioneers, said: "It's remarkable to think that this brooch this may have been a gift to Katherine from William Hastings.
"It is a true sweetheart brooch. It is likely it fell from her gown while she was strolling through the castle grounds."
It will be auctioned on 23 August with a guide price of Â£6,000 to Â£8,000.
A piece of gold jewellery linked to one of the darkest moments in Richard III's rise to power is to be auctioned.