The trafficked animals, many of them forced to stage mock kick-boxing bouts at Bangkok's Safari World theme park, will be greeted on their arrival by the wife of Susilo Bambang Yudhoyono, the Indonesian president.
They were confiscated from a private zoo near Bangkok several years ago, and DNA tests proved they had come from the island of Borneo in Indonesia.
The orangutan, being held in an animal rescue centre west of the capital, would leave Bangkok on an Indonesian military transport plane on September 23, Edwin Wiek of Wildlife Friends Foundation in Thailand said on Thursday.
The sending back announcement was welcomed on Thursday by international nonprofit groups fighting the illegal wildlife trade.
Wiek said: "It's a huge scandal and it's cost a lot of time and effort, so I'm really happy to see it coming it to an end after more than three years".
Steve Galster of the group WildAid applauded the joint action taken by Thailand and Indonesia to resolve the animals' fate, describing it as the successful outcome of a newly established Association of Southeast Asian Nation wildlife enforcement network.
"We hope that both countries learn a lesson that through further collaboration they can prevent future smuggling," he said.
An Indonesian embassy spokesman confirmed the repatriation plan, but said only 41 of the long-armed, reddish-brown primates were on the manifest, rather than the 53 mentioned by Wiek.
After a police search in 2004, Safari World's owners said their 115 orangutan were the result of a successful domestic breeding programme.
Fewer than 30,000 orangutan are thought to be left in the jungles of Malaysia and Indonesia, and environmentalists say the species could become extinct in 20 years if the current rate of decline continues.