The woman “died a short while ago in hospital” after being caught in the blast, the source said.
The powerful explosion at a bakery in central Paris on Saturday morning also killed two firefighters and left dozens more injured, several critically.
Thirty-seven others were injured in the blast, 10 of them seriously, the prosecutor's office said, after initial reports erroneously said four people had been killed.
The explosion, which smashed windows in surrounding buildings and damaged a number of parked cars, occurred at around 9am at a bakery (boulangerie) on Rue de Trévise in the busy 9th arrondissement of the city.
IN IMAGES: Paris street left devastated by deadly explosion at bakery