"I have put an end to my career to focus entirely on my studies," the 24-year-old Lefert told local newspaper Nice-Matin on Friday.
"I did not take this decision overnight. I have been considering it for a while because I always worried about my future."
Lefert, who also took a 4x200 freestyle relay silver medal at the London Games, said he wanted a bright professional future and had enrolled at business school EDHEC, one of the best in France.
"I really want to land a job which I like and not something chosen by default after my (swimming) career," he said.
"I am not Camille Lacourt, Yannick Agnel or Laure Manaudou," he said, referring to the few French swimmers who clinched lucrative advertising contracts after they won world or Olympic titles.
Manaudou, who was the 2004 Olympic 400m freestyle champion but failed to get into a final in London, gets £1.2 million a year from sponsors, according to daily newspaper Le Parisien.
By comparison, American swimmer Michael Phelps, the world's most decorated Olympic athlete, has been earning £3-4million a year since the 2008 Beijing Olympics, according to media reports.