Knut became famous after he was rejected by his mother at birth and raised by zookeepers at the Berlin Zoo.
According to one report, he was found floating in the water in the zoo's polar bear enclosure on Saturday. Bear keeper Heiner Kloes was quoted as saying, "He was by himself in his compound, he was in the water, and then he was dead ... He was not sick, we don't know why he died."
A cause of death is not known, but tests are expected to be performed Monday.
He was 4.