Witnesses reported the woman went into the wolf enclosure, where she was surrounded by the animals and then attacked, Aftonbladet reported.
Officials said emergency services personnel couldn't approach the woman when they first arrived.
"You can't just go into a pack of wolves. Police and ambulance staff couldn't get close to the victim until later," said Jan Tengeborg, Norrkoping's emergency services coordinator.
Aftonbladet said emergency services personnel were trying to sedate some of the wolves in the exhibit at the zoo near Braviken Bay, Sweden.
"It's very unusual for something like this to happen, but it has happened before," Olof Liberg, a wolf expert at Sweden's University of Agricultural Sciences, told TT, the Swedish news agency. "Zoo animals aren't afraid of humans and accidents can happen."