No one was hurt in the latest fire on Sunday morning, but all 15 makeshift homes near the Marseille port were completely destroyed, said the local fire brigade in a statement.
"Preliminary investigations suggest the fire was started accidentally," a judicial source told AFP.
Around 45 people who were in the camp will now be housed by authorities in a hotel for the next week, but their future is in doubt since the local government was on the verge of evicting them.
The incident came on the same day as 500 people gathered to pay tribute to eight-year-old Melisa, a Bulgarian Roma girl who was killed in a fire in a Roma camp in the Paris suburb of Bobigny on Wednesday.
One of her neighbours, Ahmed, described conditions in the camp as "terrible".
"We don't even have running water. This is what we want and ask for," he told AFP.
Five years ago, a seven-year-old Romanian boy, Diego, died in similar circumstances in Bobigny.
"The death of a child must not leave anyone indifferent. We must mobilise to say enough is enough. Everyone must be allowed to live in decent housing," said a tearful Veronique Decker, headmistress of the Marie Curie primary school that Melisa attended.
She led a silent procession through the town, with a banner reading: "We will remember you, Melisa".
"It is vital that we eradicate slums in a country as rich as ours. This must never happen again," said Bobigny mayor Catherine Peyge.
France has faced mounting criticism over its treatment of the Roma minority, having evicted a record 19,380 members of the community from camps in 2013.
The issue gained international attention and sparked protests after a 15-year-old Roma girl was taken by authorities during a school trip in October and deported along with her family to Kosovo.
The camp in Marseille was being targeted for eviction before being destroyed by fire on Sunday.
Local authorities said the site, which lies above a train track on land owned by the French national railway, was a health and safety risk.
"The council calls for expulsions, but offers nothing to help those who are being evicted," Jean-Paul Kopp, president of a gypsy rights organisation, told AFP.
"It is not for the local council to provide land," said Roland Blum, the first deputy of Marseille, in response. "For a start, we do not have any, and secondly, that will lead to an additional influx of Roma."