AFPA military hat belonging to Napoleon, said to have been salvaged from the battlefield after his 1815 defeat at Waterloo, is due to be auctioned in Lyon later on Monday.
The former French emperor was renowned for his "bicorne" hats, so-called because they had two points.
Napoleon's battle garb has become a focus for collectors over the years.
The red cloak he wore at Waterloo belongs to the Queen and has been in the Royal Collection since 1837.
The ankle-length, embroidered cloak was worn by Napoleon on the night before the French defeat and was looted from his carriage after the victory by Allied troops.
A man of many hats
The hat being auctioned on Monday - which was bought by a private individual in 1986 - is expected to fetch in the region of €30,000-€40,000 (£26,000-£35,000).
It is one of 19 of Napoleon's hats thought to still exist - the emperor is said to have worn some 120 similar hats during his career.
He normally had 12 hats in service, each with a three-year lifespan. They were renewed at a rate of four per year, and were first worn by valets to break them in for the emperor.
Napoleon wore his sideways, rather than with points at the front and back, so he could be spotted on the battlefield.
A similar hat - part of a collection owned by Monaco's royal family - sold at auction for €1.9m to the owner of the South Korean food and agriculture giant Harim four years ago.
That hat was apparently donned by Napoleon during the Battle of Marengo in 1800.
The hat being sold on Monday - not in as good a condition as the one sold in 2014 - is said to have been collected after the battle of Waterloo by a Dutch captain, Baron Arnout Jacques van Zuijlen van Nijevelt.
"There are also other little details: he hated the trimming and always had it removed; he requested reinforcements at certain points where he always held it," auctioneer Etienne De Baecque told AFP news agency.
Last November, a gold laurel leaf from the crown made for Napoleon's coronation was sold for €625,000.
Napoleon declared himself emperor in 1804 and waged war with other European powers, conquering much of the continent, before his final defeat in 1815.