The woman may lose an arm and was in surgery Friday at San Francisco General Hospital with injuries to both arms, Fire Chief Joanne Hayes-White said.
"My understanding is the injuries are not life-threatening, but perhaps limb-threatening," Hayes-White said.
The 3-year-old tiger, Tatiana, reached through the iron bars of her enclosure and grabbed the trainer with both front paws shortly after a regular 2 p.m. public feeding, zoo officials said.
At least 50 visitors were at the zoo's big cat exhibit, called the Lion House, said Robert Jenkins, director of animal care and conservation at the zoo. They were quickly evacuated.
The trainer has been an animal keeper at the zoo since 1997. Her main job is taking care of the zoo's four lions and three tigers, Jenkins said.
"No matter how familiar you get with these animals, they're still wild animals," Jenkins said. "You have to have a healthy respect for them and be aware of what can happen."
Tatiana arrived at the San Francisco Zoo from the Denver Zoo more than a year ago. There were no previous incidents of aggression against humans involving the tiger, said Ana Bowie, a Denver Zoo spokeswoman.
The mauling was the first tiger attack against a human at San Francisco Zoo, Jenkins said.
According to its Web site, San Francisco Zoo is one of the only zoos in the U.S. where visitors can see big cat feedings.