The 30-year-old Middleweight scored an early Wazari, which she followed up with a hold down for 19 seconds, just one second short of victory.
However, it did not prove costly as Decosse closed out the win for France's first judo gold of London 2012.
'I've waited so long, I really wanted it, now I can't tell you how happy I am. I am at peace with myself completely,' she said.
I've waited so long, I really wanted it, now I can't tell you how happy I am. I am at peace with myself completely
European champion Edith Bosch of Holland, the world number two, had to be content with bronze after being sent into the repechage following a shock quarter-final defeat to Olympic newcomer Thiele.
Bosh recorded a late Yuko to send her clash against Hwang Ye-sul, the world number six, into golden score.
She then secured another Olympic medal on judges' flags.
The other bronze went to Alvear Yuri of Colombia after she beat China's Chen Fei, who had lost a semi-final against Thiele only on the officials ruling at the end of golden score.