Zoo officials say they are puzzled by the panda's action.
"We have never had an incident like this with pandas," zoo spokeswoman Christina Simmons told The San Diego Union-Tribune.
The caretaker was bitten as the panda was being led back to her enclosure Sunday.
"Certainly there are occasionally incidents where baby animals that are being hand-raised mouth the hands of their keepers as they search for the bottle, or small birds peck at someone's hand as they get food, but an interaction such as the one that occurred on Sunday is very rare," Simmons said.
However, pandas are wild bears, she noted, and "bears are known to have volatile tempers and can be aggressive."
Bai Yun wandered out of her habitat when a safety barrier was not fully secured, and keepers were trying to get her back into her enclosure when she bit.
The bitten worker was treated at a local hospital, and California's Division of Occupational Safety and Health was investigating, zoo officials said.
The woman's right leg was "bitten and clawed," Cal/OSHA said.