The Paris prosecutor's office said a "large scale" attack was "affecting connected refrigerators in several major food retailers across France" and reported damage included "food losses and fires".
Meanwhile, Prime Minister Sebastien Lecornu instructed France's National Cybersecurity Agency ANSSI to investigate, a person in his office said.
They said France "was co-ordinating with its European counterparts, as other countries may also be affected," while also investigating whether any other French firms were at risk.
Felix Aime, a senior cyber threat intelligence analyst at Sekoia Threat Detection and Research, said Spain, Portugal, Poland, Germany and Belgium had also been affected.
Carrefour said 80 of its supermarkets were hit, while Monoprix, which operates about 490 stores in France, said five per cent of its stores were affected.
Les Mousquetaires, owner of retailer Intermarche, said fridges at about 150 of its stores were heated up to 50C for six hours overnight.
It said hackers managed to switch refrigerators and freezers to defrost mode overnight.
A spokesperson said all affected food had been discarded to eliminate health risks.
Paris resident Geert de Clercq witnessed staff at an Intermarche in central Paris throwing cartfuls of defrosted frozen foods into a giant rubbish container, with molten ice cream sloshing out of broken tubs on the street.
Cooperative U chief executive Dominique Schelcher said his supermarket had also been targeted.
"Targeting equipment essential to the preservation of fresh products and to the food supply of the French people is of extreme gravity and is unprecedented," he wrote on X.
There was no immediate estimate of industry-wide losses but Intermarche chief executive Thierry Cotillard told BFM TV the "unprecedented" attack would represent losses amounting to "tens of millions of euros nationwide in a single day".
Christel Delberghe, director general at European retail association Eurocommerce, said "This incident underlines the importance of resilience across the entire supply chain."
Supermarket cold aisles are typically run by control panels that manage an entire store's fridges and freezers and can be accessed over the internet, which lets maintenance companies adjust them from anywhere.
The source close to Lecornu said ANSSI had identified a vulnerability in software supplied by Danish company Danfoss.
Danfoss said it was "aware of incidents in France affecting a limited number of refrigeration management systems".
The affected units "were running outdated firmware," Danfoss said, adding that it had "long advised customers to keep supported systems updated with the latest firmware and to migrate legacy systems to current-generation solutions".
On August 9, cybersecurity firm Claroty's Team82 published research detailing vulnerabilities in the Danfoss AK-SM 800A controller.
According to the report, software updates issued by Danfoss addressed issues that could have allowed attackers to bypass authentication controls and execute code on compromised devices.
The attacks resemble disruptions reported at about a dozen US military commissaries in late August that led to food losses after fridges were affected.
Dutch supermarket operator Ahold Delhaize, which operates the Netherlands' largest supermarket chain Albert Heijn and has major operations in the United States, said it was also dealing with a "security issue" in its network, and had taken several systems offline to protect them.
A smartphone app for the retailer told clients that due to a "cybersecurity incident" it was unable to deliver groceries.
Online orders represent about 10 per cent of sales for Albert Heijn.
Physical stores remained open and Dutch news agency ANP reported that rivals had noted an influx of delivery orders.
Ahold Delhaize said there was no reason to see a link between the incidents in France and its supermarkets.
Ahold is the fourth-largest grocery retailer in the US, where websites for its Stop & Shop, Giant, Hannaford and Food Lion stores all had notices saying "site temporarily down".
with DPA
