The 67-year-old was wounded “above the heart” and taken to hospital in Saint-Brieuc, Brittany, where she died around midday, prosecutor Nicolas Heitz said.
Her 69-year-old partner was taken into custody and a manslaughter investigation has been launched.
While the exact circumstances of the shooting were still to be established, tests on the shooter had already established that he not under the influence of alcohol or drugs, said Heitz.
The hunting party had been crossing a field in the Goudelin area, and included around a dozen hunters and two guests, the prosecutor said.
It is the latest in a series of accidental shootings that have sparked renewed debate about the safety of hunting in France.
Every year, passers-by and hunters themselves are shot, sometimes fatally, during the season for la chasse, which many rural dwellers describe as “the wild west”.
However proposals earlier this year to ban alcohol during hunts sparked fury from hunters.