Ministers accelerated the rule change because, even though the disease has abated nationwide, there have been localized flare-ups that sparked fears of a second wave.
More than 30,000 deaths in France have been linked to Covid-19.
At the covered market at La Baule-Escoublac, a resort on France's Atlantic coast, a sign was displayed reading: "Stop Coronavirus. The wearing of a mask is mandatory."
A fishmonger at the market, who gave his name as Gregory, said shoppers had been complying.
"Since lockdown, people in La Baule were used to almost always wearing a mask, so people are respecting the rules," he said.
Hotspot in Brittany
France’s Brittany region has seen a surge in the reproduction rate of the novel Covid-19 virus for the past week.
Brittany is a popular tourist destination for domestic and international travelers.
Based on government data, the latest indicators point to signs that the transmission of the Covid-19 virus is once again picking up steam.
On July 16, the French government pushed ahead plans to make wearing face masks compulsory in enclosed public spaces amid rising new Covid-19 infections.