Curator Heinrich Klos said the cause of death has not been determined, the newspaper B.Z. reported. Knut was only 4 years old and polar bears in zoos typically live to be 30.
Knut was born at the Berlin Zoological Garden in December 2006. His mother left him and a brother on a rock in the enclosure, rejecting both cubs. The brother died of an infection soon after, but Knut was hand-reared by Thomas Dorflein, who slept on a mattress next to the bear's sleeping crate and fed, bathed and played with him for months.
The circumstances of Knut's early life made both him and Dorflein celebrities. Dorflein died of a heart attack in 2008 at the age of 44.
Klos said a necropsy will be performed on Knut to find the cause of death.
"We all held him so dearly. He was the star of the Berlin zoos," Berlin Mayor Klaus Wowereit told B.Z.