The 1.45 metre (5 feet) corpse was discovered in a stone tomb in the chapel of the Saint-Joseph convent in March last year.
The remains are most likely those of Louise de Quengo, a widow of Breton nobility who died in 1656 when she was in her 60s.
The heart of her husband, Toussaint de Perrein, was found nearby, said archaeologists at a press conference on Tuesday.
The body was found at a construction site for a future convention centre.
Four other lead coffins dating back to the 17th century were also found in the convent, along with 800 other graves, but they only contained skeletons, unlike the fully preserved Louise de Quenga.
When archaeologists arrived at Louise's tomb, they said they knew something was different.
"We saw right away that there was a lot of volume, fabric, shoes," said Rozenn Colleter, archaeologist at the French National Institute for Preventative Archaeological Research. Colleter also said that beneath the cape, they could distinguish "hands that were holding a crucifix."
After two scans and an autopsy, scientists were able to discover a bit about Louise de Quenga's medical history.
"With Louise, we had surprise after surprise," said Fabrice Dedouit, a radiologist and medical examiner in Toulouse.
An autopsy revealed "significant kidney stones" and "lung adhesions", and the heart was taken out "with real surgical mastery."
The clothes, deteriorating from years of decay, have been restored and are expected to be put on display.
Most likely choosing to live out her last days at the convent, the widow was found wearing a no-frills outfit consisting of a cape, a coarse habit, a linen shirt, cork-soled shoes, woollen breeches, a shroud over her face, and several caps.
Her corpse will be reburied in Rennes in a few months.