The 42-year-old crewman, and four others whose nationalities were not released, were aboard a lifeboat when it became detached from the ship in Marseille and "fell 10 metres" into the water, deputy mayor Julien Ruas told AFP.
Two crewmen were fighting for their lives and were evacuated to a hospital in the French Mediterranean port city, Ruas said.
He said it was not immediately known why the lifeboat had become detached.
The Harmony of the Seas arrived in Marseille early today from the Spanish island of Majorca.
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The 120,000-tonne liner operated by Florida-based Royal Caribbean only entered service in May.
At 66 metres, it is the widest cruise ship ever built, while its 362-metre length makes it 50 metres longer than the height of the Eiffel Tower.
On its maiden voyage, it sailed from the port of Southampton on England's south coast to Barcelona.
Built in the STX boatyard in the French port of Saint-Nazaire, the Oasis-class liner has 16 decks and can carry more than 6,000 passengers and 2,400 crew.
A giant climbing wall, a rope slide, mini-golf, surf simulator, floating jacuzzis, casino and 1,400-seat theatre playing Broadway musicals are among the other attractions.
RCCL has already ordered another sister ship of the Harmony to join its fleet in 2018.
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