According to Us Weekly, McDermott had sex with Emily Goodhand, 28, a woman he met while promoting Chopped Canada in Toronto.
"He told me he and Tori had a sexless marriage," said Goodhand. "I believed him."
The woman claims she spent the night with Dean at the Fairmont Royal York Hotel on Dec. 6 and Dec. 7.
Spelling, who has four children with McDermott, continues to put on a happy face for the public amidst the allegations. On Christmas Day, the actress posted a photo of their children on her blog to wish a happy holidays to her fans.
"I wanted to take a minute to wish each and every one of you a very Merry Christmas, and for those of you who don't celebrate Christmas, a very happy holiday season! As another year comes to a close, I can't help but feel so grateful for all of my blessings. My beautiful family, my amazing friends, and of course, all of you!" Spelling wrote. "We're spending the day together — cuddling, cooking, baking, unwrapping gifts, eating, watching movies and counting our blessings"
The post was signed by all members of the Spelling-McDermott household, including Dean.
There have been reports the couple have been headed towards divorce all year, but in March Spelling wrote an open letter to Star denying the rumors. In the post, Spelling wrote that upon seeing Star's report of a split, she told her children, "Daddy and I are very happy and in love. There will be no divorce and all of us will remain together as a family."