The 42-year-old and four other crew members were in a lifeboat that plunged 10 metres (35 feet) into the water during a safety drill in the southern French port.
Three other Filipinos and an Indian man were hurt in the incident, and two were fighting for their lives.
A fire service spokesman said: ‘One person is dead and four are injured, including two whose lives are in danger.’
Marseille's deputy mayor Julien Ruas told AFP the five were the only people in the lifeboat at the time when it became detached from its fastenings and ‘fell 10 metres’ into the harbour.
The city's prosecutor Xavier Tarabeux said a similar drill is carried out every week on the ship.
‘It involves putting a lifeboat to sea,’ he said.
- Sudden drop -
An investigation has been opened to find out why ‘the cable or the fastenings’ holding the lifeboat in place gave way, the prosecutor said.
The boat ‘fell suddenly into the water,’ he added.
Royal Caribbean said in a statement: ‘We are keeping our colleagues and their families in our thoughts and prayers.’
The Harmony of the Seas sailed into Marseille early Tuesday from the Spanish Mediterranean island of Majorca.
The ship had been scheduled to set sail for Italy later on Tuesday but the investigation into the lifeboat incident is expected to delay its departure until Wednesday at the earliest, the prosecutor said.
The 120,000-tonne liner operated by Florida-based Royal Caribbean only entered service in May.
At 66 metres (217 feet), 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 Southampton on England's south coast to Barcelona.
Built in the STX shipyard in the northwestern French port of Saint-Nazaire, the Oasis-class liner has 16 decks and can carry more than 6,000 passengers and 2,400 crew.
Among the onboard attractions are ‘The Ultimate Abyss’, a 10-storey slide from the top deck to the main deck which Royal Caribbean bills as the world's biggest ship-mounted waterslide.
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.
Royal Caribbean has ordered a sister ship of the Harmony to join its fleet in 2018.