Orangutans are protected and cannot be kept without a license, under the Convention on International Trade in Endangered Species of Wild Fauna and Flora.
The Phuket Zoo was found to be in the illegal possession of a female orangutan named Milo for the past two years.
When wildlife officials moved in to confiscate the animal, zoo officials said they had released her into the surrounding jungle.
"Not only were they keeping an endangered animal without a license,they released an invasive species into a protected jungle without permission," said Edwin Wiek, the head of the Wildlife Friends Foundation of Thailand.
According to Wiek, the animal had been kept in captivity for a long time and is unlikely to survive in the wild.
"We're calling for this zoo to have its zoo license removed, it clearly broke the law," he said.
The Phuket Zoo did not return calls for comments.
Orangutans are not native to Thailand. They are found on the islands of Sumatra and Borneo, where they are classified as endangered.