It was the fourth medal in five judo events for the women's team from France, although the other three medals were all bronze.
Decosse earned the gold after winning a silver medal in a lighter weight class during the 2008 Olympics. She defeated Kerstin Thiele of Germany in the championship bout.
Bronze medals went to Yuri Alvear of Colombia and Edith Bosch of the Netherlands.
In the men's 90-kilogram class, South Korea's Dae-Nam Song won the gold medal by defeating Asley Gonzalez of Cuba.
Ilias Iliadis of Greece, who earned the gold in Athens eight years ago, wound up with a bronze medal this time. Masashi Nishiyama of Japan also won a bronze.
South Korean and Russian men have both won two gold medals and a bronze in the five judo events contested to date.
2014: NFL Cheerleaders [PHOTOS]
LeBron James reveals his new Nike shoes