The victim was identified as 50-year-old Kathleen Bolton.
Police said she was standing behind the wagon putting her dog into the car when the limb measuring 10 inches in diameter fell and crushed her.
Bolton was rushed to San Francisco General Hospital, where she was declared dead at 12:14 p.m.
Winds of over 30 miles an hour were blowing at the time, but it was unclear if the winds had anything to do with the incident.
The victim's dog was uninjured.
The Associated Press contributed to this story.