The dental implant was found in a richly-furnished timber burial chamber of a young woman that was excavated in Le Chene, northern France.
The woman, who was between 20 and 30 years old when she died, had an iron pin in place of an upper incisor tooth.
It is possible the pin once held a false tooth made from either wood or bone, which could have rotted away, researchers said.
The grave was one of four adult female burials in an enclosure dating to the third century BC that were discovered during the construction of a housing development in the Champagne-Ardenne region.
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Seguin realised that the woman had 31 rather than 32 teeth, and photos taken at the excavation site showed the iron pin in the place where the missing tooth would have been.
"The fact that it has the same dimensions and shape as the teeth means that the best hypothesis is that it was a dental prosthesis - or at least, an attempt at one," said Seguin, from the Bordeaux-based archaeology firm Archeosphere.
However, most are believed to have been inserted after death to restore the appearance of the deceased.
While a post-mortem insertion of the pin in this case cannot be ruled out, researchers argued that several converging lines of evidence point to its use during life as an implant.
The findings have been published in the journal Antiquity.