It was the 200th gold medal in the history of the Summer Olympics for France.
Riner, a bronze medalist at the 2008 Olympics, was the winner of the over 100-kilogram class. It was the sixth judo medal won by French athletes in London, one fewer that the total piled up by Japan.
Riner, 23, who began winning world titles when he was in his teens, defeated Alexander Mikhaylin of Russia in the title bout and then said he planned on competing at the 2016 Games in Rio de Janeiro.
Former world champion Andreas Toelzer of Germany and Rafael Silva of Brazil received silver medals. Both lost to Mikhaylin during the event.
Idalys Ortiz won the last women's judo gold medal awarded at these Olympics in the over 78-kilogram class.
Ortiz defeated defending Olympic champion Tong Wen of China in the semifinals, handing Tong her first international loss in five years.
The silver medal went to Japan's Mika Sugimoto. Tong took one of the bronze medals, as did Britain's Karina Bryant.
It was the second judo medal for Britain in as many days.