The crown - a circular band of branches encased in a gilded, golden tube - is being displayed for three days to mark the 700th anniversary of the birthday of King Louis IX of France, who acquired it in 1239.
The relic was first mentioned by Jerusalem pilgrims in the 5th century and was transferred to Constantinople in the 10th century.
The artifact has only been displayed a handful of times in the last hundred years: in 1997, and - famously - in 1939 on the eve of World War II, to celebrate seven centuries since it came to France.
