It fell 10 metres (33 feet) into the water. Two crew members are in hospital with life-threatening injuries.
The accident happened during a stopover in Marseille, a BBC report has said.
Details of the incident were confirmed by the deputy mayor of Marseilles, Julien Ruas.
The Royal Caribbean Cruise company, which operates the Harmony of the Seas, also confirmed the death on its Twitter account.
Harmony of the Seas is the world's largest cruise liner at 362 metres long, and can hold more than 8,000 passengers and crew.
It is higher than a 25-storey building, and longer than the height of the Eiffel Tower.
It entered service in May this year.
Credit: BBC