The bear had been taking medication for advanced osteoarthritis, the zoo said in a statement. He had been moved to an off-exhibit area where he could be housed more comfortably than in the multi-level exhibit.
Kodiak bears, native to an Alaskan archipelago, are the largest species of brown bears. Toby is believed to have been born in 1990 and spent three years early in his life at the Olympic Game Farm in Washington State.
In 1993, Toby was transferred to the Grizzly and Wolf Discovery Center in Montana. He was housed with Fred, a grizzly bear, and the two bears were sent to Buffalo in 2002.
The two bears became used to being together even though they are from species that typically live solitary lives in the wild, the zoo said. Fred was euthanized in 2010.
"At the Buffalo Zoo, Toby and Fred resided in the same exhibit and delighted visitors, who loved watching them play together in their pool," the zoo said.
The zoo now has six bears, five of them approaching the end of their lives. Anana, 11, is the one exception and is to be the centerpiece of a new polar bear exhibit.
Officials said the zoo made a conscious decision some years ago not to acquire young bears until the polar bear exhibit is ready.