Koko the gorilla, who is said to have been able to communicate by using more than 1,000 hand signs, has died in California at the age of 46.
Instructors taught her a version of American Sign Language and say she used it to convey thoughts and feelings.
The abilities of the gorilla, who also apparently understood some spoken English, were documented by animal psychologist Francine Patterson.
She adopted and named pets, including a kitten she called All Ball.
"Koko - the gorilla known for her extraordinary mastery of sign language, and as the primary ambassador for her endangered species - passed away yesterday [Wednesday] morning in her sleep at the age of 46," a Gorilla Foundation press release said.
"Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication and empathy. She was beloved and will be deeply missed."
The gorilla, who was said to have an IQ of between 75 and 95, could understand 2,000 words of spoken English. The average IQ for humans on many tests is 100, and most people score somewhere between 85 and 115.
Koko the gorilla, who appeared on our cover, could chat, tease, and even argue with scientists using sign language. She has died at the age of 46. pic.twitter.com/JX9vlFzpiI— National Geographic (@NatGeo) June 21, 2018
She was born at the San Francisco Zoo in 1971. Dr Patterson began working with Koko the following year and taught her sign language, the Gorilla Foundation said.
Some scientists have cast doubt on the extent of the gorilla's communicative skills.
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However, she was the subject of many documentaries and was the cover star for magazines including National Geographic.
She also gained public attention for caring for cats.
When her tailless tabby kitten All Ball escaped and was killed by a car in 1984, Dr Patterson wrote that she had displayed grief.
Koko lived most of her life at the Gorilla Foundation in California.
She was filmed meeting the late actor Robin Williams in 2001.