The grave, crammed with Greek and possibly Etruscan artefacts, was discovered in a business zone on the outskirts of Lavau in France's Champagne region, said the National Archaeological Research Institute, Inrap.
A team from the institute has been excavating the site since October last year, and have dated it to the end of the First Iron Age — a period characterised by the widespread use of the metal.
The burial mound, 40 metres (130 feet) across, has at its heart a 14-square-metre (150-square-foot) burial chamber, not yet opened, of an ancient VIP.
"It is probably a local Celtic prince," Inrap president Dominique Garcia told journalists on a field visit.
The most exciting find, he said, was a large bronze-decorated cauldron that was used to store watered-down wine. It appears to have been made by Etruscan craftsmen from an area that is today in Italy.
The mausoleum also contained a decorated ceramic wine pitcher made by the Greeks.
The pieces "are evidence of the exchanges that happened between the Mediterranean and the Celts," said Garcia.
The end of the sixth and beginning of the fifth centuries BC were characterised by the rise of Etruscan and Greek city states like Marseille in southern France.
Mediterranean merchants, seeking slaves, metals and other precious goods, opened trading channels with continental Celts.