The website, Gawker.com reported on Tuesday that it received a message from someone claiming that he or she signed into Romney's Hotmail account.
The tipster said it happened by correctly guessing the answer to a security question about Romney's favorite pet. The hacker also said that Romney's account on DropBox, a file-sharing service, was also accessed.
Romney's campaign spokeswoman refused to say whether Romney still uses the Hotmail account.
The Associated Press contributed to this report.