Delhaize said a "computer incident" affected about 40 of its shops in Belgium and Luxembourg, and Carrefour said 15 of its Belgian stores were hit.
Several ICA and Coop stores in Sweden and 13 SuperValu shops in Ireland were also affected.
Belga News Agency reports the Belgian attack began around midnight on Sunday night.
No one has been identified as responsible.
Food worth millions of euros has reportedly been thrown away, and images show staff discarding trolleys of spoiled goods.
A Delhaize branch in Hoeilaart told Het Laatste Nieuws it lost €100,000 of produce, not counting the cost of closing the shop.
RTL Info reported that temperatures at affected Delhaize stores climbed to 15 C, and some stores brought in refrigerated trucks to save stock.
An ICA store in Skövde said its freezers had been set to 50 C (122F).
Carrefour said all its affected stores stayed open, while Coop said seven of its 800 Swedish stores were affected and the situation was contained.
ICA said the stores involved operate outside its central network.
Belgium's and Sweden's national cyber-security agencies said they were monitoring the incident.
Belgian refrigeration fitter Sabcobel estimates at least 20 of its units there were affected. It says the attackers triggered the freezers' built-in defrost mode and overrode its temperature and duration settings, the same method used in France. CEO Stephan Moelans said the company has worked "very hard" to protect its units, but that fixing the vulnerability requires physical modifications. The simplest safeguard is moving units onto a secure internet connection, which most customers did after the French attack. Delhaize's parent, Ahold Delhize, said it is investigating and had recently dealt with a separate cyberattack that disrupted IT systems, supply chains and deliveries at Albert Heijn and other brands. Tom's Hardware links the rise in cyber intrusions affecting the physical world to AI agents, meaning software that can act autonomously.
