The woman, 38, was hit in the forehead by the stone, which was thrown by one of the five gorillas at the Kolmården National Park in central Sweden. The stone measured roughly five centimetres in diameter, said Marjorie Castro, head of the zoo.
"At first everything seemed fine. She was bleeding a little from her forehead, but seemed to be in good health," Castro told the Aftonbladet paper. "But she was hit in the head and gorillas have enormous strength, so we called an ambulance. After a while, she felt weak and we had to lie her down."
Castro added that the gorillas have never thrown things at visitors before, with the exception of lighter objects like grass.
However, following the news of the stone-throwing gorilla, other Swedes shared similar stories about their experiences with the Kolmården primates. The mother of a 7-year-old boy told Aftonbladet that Enzo the gorilla had thrown a stone at her son in July last year.
"I got so scared and was thinking 'Please, don't throw rocks," the boy told the paper. "They are scary."
The gorilla is the largest primate in the world and is native to the forests of central Africa. They are the closest relative to the human being, after the chimpanzee and the bonobo. It is estimated that fewer than 700 mountain gorillas are left in the wild.
Kolmården made headlines in June last year when the zoo's wolf pack mauled and killed a female employee.