The artefact was found in a latrine as Fontainebleau Palace was being prepared for restoration.
Officials said it was the first time such a possession has been found. Catherine was queen of France from 1547-59
The three-and-a-half inch pin bore the initial “C” and the white and green finish known to be Catherine’s colours. Conservator Vincent Droguet said the pin’s location in a communal toilet – as opposed to a private one – was a “mystery”.
He added: “Perhaps it was stolen, and just fell in.”