Video footage shared on social media showed the orangutan struggling in the water before sinking below the surface at the Safari Phu Quoc in the southern island on June 24.
A zoo employee threw in a life buoy before jumping into the pool himself to save the orangutan. He managed to bring it ashore and performed chest presses as first-aid.
The orangutan has survived and is in stable health now, a zoo representative said.
The zoo has erected signs to warn visitors not to feed animals, except certain species. However, many ignore the warnings, the representative said.
The Safari Phu Quoc is a semi-wild zoo spanning 380 ha. It is home to around 3,000 animals, including 175 species of birds, mammals and reptiles.
A video posted on Facebook shows an orangutan almost drowned trying to get food thrown by a visitor into a pool in Phu Quoc Safari.