The 42-year-old Filipino crew member died when a lifeboat, with five crew, became detached from the fifth deck during a safety exercise, the BBC reported.
Two crew members are in hospital with life-threatening injuries.
The accident happened during a stopover in Marseille.
The Royal Caribbean Cruise company, which operates the Harmony of the Seas, confirmed the death on its Twitter account. "We're keeping our colleagues and their families in our thoughts and prayers," the company said.
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, 2016.
Among its luxurious facilities are 20 restaurants, 23 swimming pools, a theatre and a casino.