Stephen Ellison, 61, British consul general in Chongqing, was walking by a river in the nearby ancient town of Zhongshan when he heard a woman screaming, he said in a BBC interview.
He said the woman had slipped into the river, and “it was pretty quickly obvious that she was having problems, that she couldn’t swim”.
“There wasn’t a lot of time to do anything other than get in,” Ellison told the BBC.
The British Embassy said she was “immensely proud” of Ellison.
“His courage and commitment demonstrate the best of British diplomats in the world,” tweeted Foreign Minister Dominic Raab.
The Chinese state-run CGTN reported that some onlookers took Ellison home after the incident. He received dry clothes, coffee and food.
They also presented her with a 6-meter (20-foot) paint roller from the ancient city of Zhongshan, CGTN reported.
After the student recovered, she and Ellison had a drink together, the diplomat told the BBC. She invited him to a meal and Ellison said he hoped to accept soon.
Reuters additional reports.