When the 40-year-old '90210' star married McDermott in 2006, she failed to protect her fortune, reported Radar online.
"Dean offered to sign one, but Tori refused. She truly believed Dean was her soul mate and there was zero need for a prenup," a source said.
"With the recent allegations that Dean cheated on Tori, she is now kicking herself for not securing the prenup with Dean. She doesn't plan on divorcing Dean, but feels that a prenup might could have prevented him from cheating on her," the source added.
Her mother, heiress Candy Spelling, is said to be sitting on a staggering USD 600 million.
Spelling married McDermott, 47, soon after his split from Canadian actress Mary Jo Eustace and her divorce from first husband Charlie Shanian was finalised - a man who she did have a prenup with.