The animal, which appeared to have been hit by a ferry propeller, lurched breathing heavily into Skippys Fast Food in Molde, near Ålesund, a large gash on one leg and damage to its face.
The restaurant's owner, Mohamed Rashed, rang the police in panic.
"We did not know what kind of animal it was. I have never seen an animal like that in my life," he told TV 2. "I was terrified it was going to attack."
When the local wildlife officer, Håkan Sunde, tried to put the animal in a crate, it lunged at his finger, causing him to bleed profusely.
"I didn't have any gloves, so when I put the otter indie the dog cage, it decided to start eating on one of my fingers," Sunde told The Local. "It was not a serious injury, but it was quite strong in the jaws. I had a tetanus shot just after the bite, so it's OK."
He said he was seeing more and more otter-related activity in the region.
"The amount of otters is growing at the moment, because we observe them often and they show up in strange places," he said.
The otter had to be put down as it was too severely injured to be treated.