The woman was injured in the face and on one arm after mauling from the dog, a pitbull terrier, near her home in Kristinehamn.
Medical staff were shocked by the woman’s injuries.
“I have never seen such serious bites before,” a surgeon told local newspaper Nya Wermlands-Tidningen.
The ex-boyfriend was arrested on Sunday on suspicion of serious assault, according to SVT.
Another man, reported to be a friend of the ex-boyfriend’s, was arrested on suspicion of not reporting the crime. He allegedly knew that the offence had taken place but said nothing to the police.
The dog is being cared for by a neighbour.