Knut died in front of hundreds of visitors on Saturday after wading into the water in his enclosure and having a spasm.
His death at such a young age took zookeepers by surprise.
Polar bears live up to 20 years in the wild and usually live even longer in captivity because they are not exposed to hunger, thirst or infections.
Knut was rejected by his mother at birth, along with his twin brother, who only survived a couple of days. He attracted attention when his main caregiver, Thomas Doerflein, camped out at the zoo to give the button-eyed cub his bottle every two hours, and went on to appear on magazine covers, in a film and on mountains of merchandise.
Doerflein died in 2008 of a heart attack, earning front page headlines in German newspapapers as "Knut's daddy."
A necropsy for Knut is scheduled for Monday to figure out the exact cause of death.
Hundreds of fans flocked to Knut's zoo enclosure on Sunday to mourn his sudden death, laying down red roses and white stuffed polar bears, lighting candles or putting up pictures of Knut with personal messages for him. Many sobbed and shared their memories.