The boy crawled through a barrier into the enclosure and fell into a moat around 4 pm (1600 GMT), zoo director Thane Maynard told reporters.
The gorilla, a 17-year-old male named Harambe weighing more than 400 pounds (180 kilograms), “went down and got him,” he said.
Witnesses said the gorilla dragged the screaming boy around the habitat, local media reported.
The zoo’s dangerous animal response team shot and killed the gorilla around 10 minutes after the boy first entered it.
He was hospitalized with injuries that were not life-threatening, media quoted the police as saying.
The zoo’s response team decided to shoot the gorilla rather than tranquilize him because a tranquilizer would not have taken effect immediately, Maynard said.
“It seemed by our own dangerous animals response team to be a life-threatening situation,” he said. “They saved that little boy’s life.”
The zoo houses 11 gorillas, according to its website.
“We’ve never had a situation like this,” Maynard said.
Pictures courtesy: The Daily Mail