A video she posted on Facebook as Sagungan Batngulueam on Tuesday was viewed more than 900,000 views in less than a day.
The endearing video shows her beside the cage where the monkey is now kept at the foundation in Phetchaburi. It shows the animal reaching out to hug and kiss her, both of their faces pressed against the bars.
The teen explained that her family had raised the orphan monkey but ultimately decided it should have simian companionship and gave it to the foundation.
On visiting her former pet on Tuesday, the girl learned that it ate little, had lost weight and was missing its tail.
She initially surmised that it had chewed off its own tail due to anxiety and loneliness but was told another monkey had bitten off the tail.
She asked for the monkey to be returned to her care and was refused, so she made the online appeal for public support.
In an update on Wednesday, the teen said the foundation had explained that the monkey could not be returned because it had already been registered as a wild animal under protection.
The foundation suggested her family register with the National Parks, Wildlife and Plant Conservation Department for permission to raise a wild animal and they might be able to recover the monkey.
The teen said her family would do anything to get the monkey back.