The armed robber at the Carlton Hotel in Cannes targeted a temporary jewellery exhibition it was hosting from a diamond house owned by Israeli billionaire Lev Leviev.
It remains unclear how many pieces were stolen.
The valuable jewels were due to be on public display until the end of August.
More top news
Daughter whose mother was left for hours on a hospital trolley in A&E has campaigned tirelessly for change - and with some success.
After unseasonably windy weather with gales and big battering waves along exposed western coastal spots, a little calmer into tomorrow.
Sinkhole causes two lanes to collapse but luckily no injuries have been reported.