The 37-year-old man jumped over a fence into a water-filled ditch at the edge of the bear's enclosure Monday morning, police said in a statement.
Zoo keepers, who had just let Knut into his outdoor enclosure, were able to lure the bear back into his cage by producing a leg of beef.
Police said the man, a German, was less cooperative, initially ignoring instructions to leave the enclosure. He was led away unharmed but, although he was soaked and cold, he refused to undergo a medical checkup.
Police said that, before being let go, the man told them that he felt lonely and the bear appeared lonely, too.
Knut, now age 2, was hand-raised after his mother rejected him at birth. He rose to stardom early last year as a cute white ball of fluff, but has since grown rapidly into a hulking 440-pound predator.