A spokeswoman for Berlin police said Thomas Doerflein was dead when authorities arrived at the apartment and that they had no information on the cause of death.
Doerflein was 44 years old.
He gained fame in Germany and beyond as the ever-present caretaker for Knut, a polar bear cub abandoned by his mother in late 2006.
Knut became a worldwide sensation when the Berlin Zoo decided to raise him by hand, and Doerflein was there for every stage of the bear's progress.
With his burly build, beard and ponytail, Doerflein was a distinctive figure at the side of the growing bear. He nursed young Knut in his arms behind closed doors and wrestled with him after the bear grew old enough to play.
When Knut made his public debut in March 2007, Doerflein was at his side. They started a daily performance for the thousands of visitors who flocked to see the bear at his outdoor enclosure.
But the "Knut Show" ended in July of that year when the zoo's director ruled that the bear had grown too large for Doerflein to frolic with in safety.
The boisterous bear now weighs more than 265 pounds, has his own feature-length film, blog and TV show. He has graced the cover of German Vanity Fair and appeared on a set of stamps.
The Berlin Zoo credited Knut with a 27 percent increase in visitors in 2007 and profits of nearly €6.8 million ($9.9 million). It has licensing agreements for all kinds of Knut products, including stuffed animals, T-shirts, mugs and DVDs.
Last November Doerflein was awarded Berlin's medal of merit for his service to the city - and to Knut.
Doerflein worked at the zoo for more than 25 years. According to the newspaper Die Welt, Doerflein was a Berlin native with two grown children. He lived with his girlfriend and her young son.
By Associated Press Writer Patrick McGroarty