The monkey was found Saturday night -- a day after he disappeared from the zoo -- at a park by a passer-by who spotted the primate in the bushes, the San Francisco Police Department said in a statement.
The passer-by coaxed the monkey from the bushes into his backpack, and then called the authorities, according to the statement.
Police officers brought the monkey to the zoo, where officials determined it was the missing primate.
"Although hungry, trembling and thirsty, Banana-Sam is currently safe back," the zoo said in a statement.
Zoo workers discovered the theft Friday morning, when they found a back perimeter gate had been breached and two holes cut in the mesh fence of the squirrel monkey exhibit.
The male monkey, known as Banana-Sam to his keepers, is age 17, over 12 inches tall and weighs about 2 lbs. Corrine MacDonald, the curator of primates and carnivores, told CNN affiliate KGO earlier Saturday that squirrel monkeys can live into their 20s, saying his relatively advanced age could put him at more risk.
Banana Sam' keepers warned the public that while he looks very cute, he is not a pet -- and can deliver a nasty nip. The curator said such monkeys carry diseases and can cause serious infections if they bite a human.
The monkey needs a specialized diet to stay healthy, the zoo says.
What motivated the thieves to swipe the monkey is not clear. Common squirrel monkeys are not endangered, and they can be found at pet trade markets -- with such sales illegal in California, the zoo says -- or medical research institutions.
MacDonald said the zoo's other squirrel monkeys were visibly shaken by Banana-Sam's apparent capture. She said the decision was made to pull them off the exhibit due to concerns about the integrity of the exhibit and that a "copy-cat" could try to take other monkeys.
It was not immediately known when the zoo planned to reopen the exhibit.
An unknown person was quick to set up a fake Twitter account in Banana-Sam's name, following in the path of a cobra that escaped at New York's Bronx Zoo in March. The snake's mock Twitter account, with humorous tweets on its supposed whereabouts in New York City, swiftly attracted a large online following.
Under the handle @SF_BananaSam, the "monkey" is now tweeting his way round San Francisco.
"Went to monkey bars in Golden Gate Park playground, left disappointed. #nobananadaiquiri" one post reads.
Another says: "I'm a funny-looking vegan who ran away from home and who people follow on Twitter. IN other words, A NORMAL SAN FRANCISCAN."
