The western lowland gorilla passed away in her sleep Tuesday morning at the Gorilla Foundation's preserve in California's Santa Cruz mountains, the foundation said.
"Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication and empathy," the foundation said on its website. "She was beloved and will be deeply missed."
Koko was believed to have had an IQ of between 75 and 95 and could sign more than 1,000 words. The average IQ of a human is around 90 to 110. She also understood spoken English.
Born at the San Francisco Zoo, the gentle, nearly 300-pound ape began learning sign language as a baby in 1974 from Dr. Francine "Penny" Patterson as part of a Stanford University project.
She spent her entire life in captivity and became a celebrity in her own right.