The man was spotted at dawn by a fishing boat on Tuesday morning as he attempted to cross the busy shipping lane which separates the two countries.
Charities say migrants in northern France are increasingly desperate and willing to take greater risks to reach Britain due to hardline policing and increased security at ports in the area.
From late last year, the French coastguard reported a surge in attempts by men and women to cross the Channel in rubber dingies as well as in stolen boats.
In a statement Tuesday, French maritime authorities said that a coastguard speedboat had been dispatched to rescue the swimmer who was about five kilometres off the coast of Calais.
“Suffering mild hypothermia, the migrant was handed over to the SAMU (emergency services) and border police in the port of Calais,” the statement said, without giving his age or country of origin.
The northern Channel coast has long been a magnet for refugees and migrants who gather there hoping to board trucks and ferries heading to Britain.
Last month rights group Amnesty International accused French authorities of harassing and intimidating charity workers who attempt to help migrants there, many of whom live in squalid camps.
A mayor in the small port of Ouistreham was handed a six-month suspended prison sentence on July 2 for harassing people helping the migrants by wrongly issuing parking fines for their vehicles.
Many locals on the northern coast complain about the impact of squalid migrant camps on the image of the area, while trucking companies say they face major security problems.