Carl Friedman, director of animal care and control for the city of San Francisco, said the man hopped the fence mid-afternoon Monday and climbed down a small slope to a large barrier that separates the animals from visitors.
A witness notified zoo employees, and animal control officers responded and cited the man, who was with two female companions.
Friedman said the man had not scaled the second barrier and did not make contact with the rhinos. No one was hurt.
"(He) didn't actually get into the exhibit with the rhino but very close to it," Friedman said. "Why that person did it is beyond me."
Friedman said he had heard varying accounts of why the man climbed over the fence, including that he wanted to take a picture and that he was attempting to retrieve a peacock feather.
It did not appear the man was intoxicated; rather he "just did a real stupid thing and it seems to me he might have realized that," Friedman said.
Friedman said that since the fatal tiger mauling at the zoo last Christmas, signs have been posted throughout the zoo asking visitors who see any unusual activity to contact zoo employees.
"The public are sort of our eyes and ears," he said.
Friedman served on the peer review team created to review zoo safety features and protocol following the tiger attack.
The man who was cited Monday will pay a fine and may face jail time, Friedman said.
Two rhinos were in the enclosure at the time, 37-year-old "Ellie" and 21-year-old "Gene," Friedman said.