He had been missing since Saturday.
The body of the 12-year-old boy was found by emergency services in Gard, the local prosecutor announced this Friday.
He was the last member of a family swept away in their vehicle to be searched for.
His sister, aged 4, was found in Gard by the gendarmes on Tuesday.
The body of a man discovered Monday has been identified as the father of the two young victims.
A woman, the girl's partner and mother, is the only survivor of the tragedy.
Their car was swept away on Saturday evening around 11:30 p.m. while trying to cross a submersible bridge over the Gardon.
Their vehicle had entered after signs had already been installed to warn of the danger and prohibit passage, but before a barrier was put in place to block access to motorists, detailed the authorities.
A man died in Ardèche
Another body was discovered Tuesday evening in Ardèche, downstream from the place where a man disappeared after severe weather over the weekend, but emergency services were unable to access it, the department's prefecture said.
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The body was reported by hunters and relatives of the man missing since Saturday evening in Saint-Martin-de-Valamas, the prefecture told AFP.
“The location is currently inaccessible by police and fire units” and the body could not be identified, she continued.
A new attempt to access it must take place on Wednesday morning.
According to a gendarmerie source, the missing person is the manager of a hydroelectric power station who had gone to check its installation.