Dr Yves Cohen told French media a swab taken at the time was recently tested, and came back positive for Covid-19.
The patient, a man in his 50s who has since fully recovered, said he has no idea where he caught the virus as he had not been to any infected areas.
This news means the virus may have arrived in France almost a month earlier than previously thought.
Until now, the country's first three cases of coronavirus were confirmed on 24 January.
Of those, two had been to Wuhan in China - where the outbreak was first detected - and the third was a close family member.
How was the new case found?
Dr Cohen, head of emergency medicine at Avicenne and Jean-Verdier hospitals near Paris, told French broadcaster BFMTV that he recently went over the files of patients admitted with flu-like symptoms in December and January.
In total he tested the swabs of 14 patients, he said.
"We re-tested the nasal swabs which were conducted at the time in relation to another diagnosis, to try and find traces of coronavirus," he said.
"Out of 14 patients, one tested positive. We tested it two more times to make sure there was no mistake. And twice, it came back positive."
Dr Cohen said he had alerted the National Health Agency (ARS), and was urging other virologists to re-test swabs in their hospitals for Covid-19.
A full report is due later thiFrance's first Covid-19 case was in Decembers week, and will be published by the International Journal of Antimicrobial Agents, he added.