The Lincoln Park Zoo in Manitowoc closed after the incident Friday morning. Police say the 47-year-old woman lost a thumb and a forefinger, and two other fingers were partially severed.
The woman's boyfriend was bitten as he tried to pry the bear's mouth off her hand, but he didn't lose any fingers.
Her 3-year-old granddaughter wasn't injured.
A mayor's office statement says alcohol played a factor.
It's unclear which of two Asiatic black bears bit the women when she put her hand through their enclosure's fence.
Police tell the Herald Times Reporter the bears likely won't be euthanized.
Manitowoc is about 80 miles north of Milwaukee.