Kroger was one of the first companies to let customers know about the breach. Three more retailers Best Buy, Tivo and Walgreens have also come forward saying their customers were also affected.
All of the companies use the same marketing firm Epsilon.
Officials said hackers accessed Epsilons customer files. The companies use Epsilon to send out e-mails to customers.
Epsilon has said no personally identifiable information is stored in the files its mainly just e-mail addresses. But companies are warning customers to be wary of e-mails asking for personal information.
In an e-mail to customers, Kroger assured shoppers the only information at risk is their names and e-mail addresses, but warned them not to open any e-mails from unknown senders.