The Zoo pulled out all the stops to fete its star attraction, who has gone from a fluffy white powderpuff of a cub to a full-grown polar bear with a cheeky curiosity and a penchant for muddying his coat by rolling in the dirt.
"He's so dirty! They should have given him a bath," said 5-year-old Leni Schmidt, who came with her mother and a friend to watch Knut munch his birthday treat of fresh fruit, vegetables and rice topped with a toy wooden candle.
The zoo estimated that some 2,000 people had visited the birthday bear by noon and a steady stream were still heading toward his pen, despite the cool weather and overcast skies.
Yet Knut himself seemed more interested in playing with his toy candle and licking the crumbs from the platter that bore his birthday treat than the crowd of well-wishers.
The zoo said the Knut craze has generated up to Ђ10 million (US$14 million) over the past year through ticket sales that have more than doubled and the licensing of Knut toys, books and other trinkets.
Among fans Wednesday was Sabine Haelter, wrapped in polar cub-imprinted scarf, who had traveled from central Germany to be at the zoo for the event, and Berlin native Marion Maetzeld a member of the zoo who visits regularly.
"We wish him a huge polar bear cake and, later a nice lady-bear as a mate," said Maetzeld, snapping pictures of Knut from her perch on a boulder.
Weighing just 810 grams (nearly 2 pounds) at birth, Knut has bulked up to more than 110 kilograms (243 pounds), and in July had to be put on a diet because of concerns that treats like his favorite croissants were piling on the pounds.
That means he won't be allowed a slice of a giant birthday cake baked in his honor made from 300 eggs, 15 kilograms (33 pounds) each of flour and sugar, as well as 10 kilograms (22 pounds) of marzipan. Slices were sold to onlookers and will go to the zoo's foundation.
Abandoned at birth, along with his twin brother, who only survived a couple of days, Knut first attracted attention when Berlin's media picked up the story of his main caregiver camping out at the zoo to give the cub his bottle every two hours.
The story angered animal rights activists, who tried to sue the zoo for cruelty to animals by preventing nature from taking its course when the cub was rejected even if it cost him his life.
By that time, Knut was a 3-month-old white fur ball, with button eyes and his own fan club, who had been photographed dozens of times including for the cover of Vanity Fair and attracted hundreds of thousands to the zoo.
Now that he is grown, zoo officials say they do not have room to keep Knut permanently and are considering other offers from zoos in Europe, with an eye to finding him a mate.
Thomas Doerflein, who raised Knut by hand, cuddling him and playing with him in addition to feeding him from a bottle, told Germany's mass-circulation Bild on Wednesday, he hoped the bear would soon find a new home in another zoo.
"A spacious enclosure. A female partner. At some point, Knut needs to leave me," Doerflein was quoted as saying.