Bai Yun, whose name means "White Cloud", is on long-term loan to the zoo from China.
Since 1999, Bai Yun has delivered three female and two male cubs, all but one fathered by her companion, Gao Gao. The sex of the new cub will not be known for several months, Simmons said.
The baby was born around 2.10pm PDT (9.10pm GMT) on Sunday, after about three hours of labour – longer than normal for Bai Yun, Simmons said.
"When they heard the cry from the baby, the veterinary staff was relieved for the mom and delighted for the baby," Simmons said. "It is about the size of a stick of butter and pandas are born hairless and pink."
Bai Yun and the new cub are in seclusion during what zookeepers call "a critical bonding period." As long as everything appears to be going well, zoo veterinarians leave the mom and cub alone, Simmons said.
"Panda moms and cubs go into their dens and the babies stay there for a long time," Simmons said. "We don't expect the baby to emerge before December."
The zoo does have a closed-circuit camera in the den, and people can watch the activity on the San Diego Zoo Panda Cam.
The cub's father, Gao Gao, is not spending time with Bai Yun, Simmons said.
"Gao Gao is not particularly aware of what's going on," she explained. Pandas are normally solitary animals – they only associate with other adult pandas for breeding."
The birth of the cub brings San Diego Zoo panda count to four: the parents, a three-year-old son named Yun Zi and the new cub. That's more than any zoo in the world outside China, Simmons said. Four of Bai Yun's other cubs were later sent on to China.
"We work very closely with the Wolong Giant Panda Research Centre in China, and our good fortune has a lot to do with our intense collaboration with them," Simmons said. – Reuters