Julien Moreau, 27, started the swim on Tuesday and finished the 38.5 mile (62km) journey at Sillon beach on Saturday.
He said he was doing the swim to raise awareness of ocean pollution.
In 2015, Wendy Trehiou, from Jersey, swam the distance in the opposite direction from St Malo to Jersey.
Mr Moreau wrote on the Julien Moreau - Adventures : "Everything is possible, and now it's official, I am the first and only person to swim from Jersey to Saint-Malo.
"I am very proud of the team â€ª#â€Žswim4oceanâ€¬. We worked with intelligence, cohesion and determination."
Mr Moreau started the challenge on Tuesday and swam six to eight hours a day before resting on board the Sea Shepherd boat that accompanied him on his journey.
The most popular Channel swim is the 21 miles (33.8km) route between Dover and Cap Gris Nez.
A French man is thought to have become the first person to swim from Jersey to St Malo, in north-west France.