The title came after a mail carrier alleged she was bitten on the ankle by the pint-sized canine on Aug. 23.
On Wednesday, the Windsor Licensing Commission rejected an appeal to the designation by Molly's owner, Jason Scott.
Scott says he's not impressed by the ruling.
As a consequence of the designation, the Chihuahua has to be muzzled when it's outside, and have a microchip implanted that identifies Molly as a dangerous dog.
Scott must also get at least $1 million in insurance coverage.
"It's a joke," said Scott, adding that Molly is "the size of a shoe."