Tong's gold was China's fourth in the judo competition
China's Tong Wen pulled out a big throw in the closing seconds to defeat defending champion Maki Tsukada and take gold in the women's 78kg category.
There was just 16 seconds left when the three-time world champion threw her Japanese rival to overturn the deficit.
Slovenia's Lucijia Polavder and Cuba's Idalys Ortiz took the bronze medals.
Karina Bryant lost in the first round to end British hopes of a judo medal. The four-time European champion was beaten by Mexico's Vanessa Zambotti.
Zambotti's subsequent defeat to defending champion Maki Tsukada confirmed Bryant would not be given a second chance in the repechage.
"I'm very frustrated because I didn't perform to my full potential," said Bryant, competing in her third Games.
"Zambotti made things very difficult, but I should have been on better form. If I had performed to my full potential she wouldn't have given me any problems.
"Going into the bout I felt like I hadn't left any stones unturned. I wouldn't have changed anything about my approach and I came in feeling very strong mentally and physically."
Bryant's exit completes a disappointing Games for the British judo team, with medal hopes Euan Burton and Craig Fallon also falling short.