Tiny Kamiko Dao Tsuda-Saelee was playing at her family home in Las Vegas when the dog attacked her.
Her heartbroken mother, Layla Tsuda, said she left Kamiko alone with the dog for a few seconds while she nipped to the bathroom.
But she returned to the harrowing scene of the dog, named Mana, savaging her baby daughter.
Mrs Tsuda managed to stop the attack but Kamiko had already suffered serious injuries.
She was rushed to hospital with blunt force trauma to the head but could not be saved.
In a statement, devastated Mrs Tsuda told KHON-TV: “I have no words to explain my pain.”
She described the Mana as a good family pet who had been with the family for nine years and had never bitten anyone.
The dog was destroyed by Clark County Animal Control a few hours after the attack.
Officials said: “The owner surrendered the dog to the County. A dog that has attacked and killed a person would not be allowed to be adopted.”
Police said no charges were expected to be brought against the family as it was a tragic accident.
Las Vegas police officer Larry Hadfield said: “It’s an absolute true tragedy.
“Regardless of how trusting you are of animals you have to make sure you are supervising.
“You have a family that’s going to be grieving for today and the rest of their lives.”