A police spokesman said: "The young man was in the water with two friends, when a shark attacked him, biting his right thigh, and his groin area.
"The victim was pulled out of the water, and emergency workers arrived very quickly. Despite cardiac massage, he died within half an hour of the attack."
The beach was packed at the time, and members of Dubosc's family were among those who watched the horror unfold.
Frederic Carre, a local sub-prefect, said members of a medical-psychological emergency unit attended the scene, and were treating many of the witnesses.
Carre said there had been a huge amount of "anger and emotion" when people saw what had happened.
Dubosc, who lived in Saint-Leu, on the west of the island, was an extremely experienced bodyboarder, and would enter the water at any opportunity.
Today France Meteo, the national weather service, had warned of strong swells off Reunion, which is one of the country's overseas territories.
On February 21st Dubosc's close friend Alexandre Naussac, 26 and another trained shark spotter, died on a nearby beach that had also been officially closed to watersports.
The attack happened in Saint-Andre, and saw Naussac being bitten in the femoral artery.
The thigh wound caused blood to pour out of Naussance, as those he had been bodyboarding with also desperately tried to save him.
A helicopter arrived with a team of medical workers, but "by the time he was pulled on to the beach he was dead", an emergency worker said at the time.
The latest death marks the 21st shark attack on the Indian Ocean island in just six years, with nine of the incidents proving fatal.
Dubosc's death will intensify the shark crisis on Reunion, which is by far the most dangerous place in the world for shark attacks.
The island, which is just 40 miles long, has seen some 15 per cent of all the world's fatal attacks over the past five years.
The French authorities have insisted they are tackling the problem with nets and boat patrols, and by catching and killing about 100 sharks a year.
In turn, conservationists have complained about such culls, saying that there are enough of the fish being killed by pollution.