Manaudou has not swum competitively since the Paris Olympics two years ago, and many expected the announcement to come during the European Championships at home.
In total, he has won 20 championship gold medals during his career.
"Two years after the Paris Olympics, I announce the end," he told French television on Thursday evening.
Manaudou was awarded France's highest honour, Knight of the Legion of Honour, after winning Olympic gold in the 50-metre freestyle at the 2012 Olympics.
That gold meant he and his older sister Laure Manaudou made history as the first pair of siblings to win Olympic gold in swimming. She won the 400-metre freestyle at the 2004 Olympics.
Florent Manaudou will be celebrated ahead of the final session of the European Championships in Paris on Saturday. He won two bronze medals at his home Olympics in 2024, in the 50-metre freestyle and 4x100-metre medley.
