The bicorne black beaver felt hat was valued between €600,000 and €800,000 (£525,850-£701,131).
The person who bought the hat has not made themselves known.
Historians say the hat was part of his brand. Wearing it sideways made him recognisable in battle. He owned about 120 bicorne hats over the years.
However, only 20 are thought to remain - many in private collections.
The hat is being sold along with other Napoleonic memorabilia assembled by an industrialist who died last year.
But the auctioneers said for specialists, the hat is the true holy grail.
The emperor wore his hat with the corns parallel to the shoulders - known as "en bataille" - whereas most of his officers wore their hats perpendicular to the shoulders.
Auctioneer Jean Pierre Osenat said: "People recognised this hat everywhere. When they saw it on the battlefields, they knew Napoleon was there.
"And when in private, he always had it on his head or he had it in his hand, and sometimes he threw it on the ground. That was the image - the symbol of the emperor."
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The auctioneers said this hat comes with impeccable provenance, remaining throughout the 19th Century in the family of Napoleon's palace quartermaster.
The hat being auctioned by Osenat auction house in Fontainebleau has a cockade that Napoleon fixed to his hat in 1815, during the crossing of the Mediterranean from his exile in Elba to Antibes, where he led a brief return to power.
Other items being sold include a silver plate looted from Napoleon's carriage after his 1815 defeat at Waterloo and a wooden vanity case he owned, with razors, a silver toothbrush, scissors and other belongings. - Reports BBC news