Nacho was 15, the zoo said. He suffered from epilepsy and had other physical problems and would probably have died much earlier in the wild.
After a veterinarian found a large tumor near his heart, the zoo euthanized Nacho on Thursday, The Orange County Register reported.
At the zoo, Nacho was housed with Yo-Yo, a female black bear. The bears were named in a contest for children.
"I can still remember the day when Nacho and Yo-Yo were introduced at the OC Zoo," Orange County Supervisor Todd Spitzer said. "They were among my children's favorite animals there, and we'll all miss seeing Nacho there very much."
The California Department of Fish and Wildlife sent Nacho to the zoo in 2001. He had been orphaned as a cub and was raised in captivity because he could not survive in the wild.