Tak said in the popular photo-sharing application that their engagement party will be on Nov 19.
The voluptuous actress, no stranger to Thailand's front pages, is known for her erotically charged roles in films, such as "Jan Dara", "The Judgement" and "Bang Rajan".
Earlier this year, the 27-year-old outspoken celebrity sparked a widespread outcry when she posted a message on her Facebook profile page, expressing harshly worded contempt for Ampon Tangnoppakul, a lese majeste convict who died in prison, and his followers.
Mr Boonchai, the father of three daughters, has been married five times. His fifth ex-wife is also a former actress, Benja Baramee.
The Dtac chairman and his family ranked 13th in Forbes magazine's "Thailand's 40 Richest 2012" released on Oct 29 with assets worth US$990 million (3 billion baht).
He also invested in the two-billion-baht Museum of Contemporary Art on Bangkok's Vibhavadi Road.
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Thanapa Kongmalai, the mother of the actress, said she was impressed with Mr Boonchai's "sincerity" and "gentlemanliness" because he showed his divorce certificate to her when he asked her permission to marry her daughter.
"Personally, I'm very proud of him [Mr Boonchai] because he likes to make merit, like me.
"He also has good manners and is down to earth and I've found out that even though he's a billionaire he's not arrogant or pretentious.
"I want a good man to take care of my daughter, not a wealthy man, because I'm already rich," Ms Thanapa said.
Asked about the 31-year age difference between the couple, she said it would not be a problem.
The bride-to-be's mother said Mr Boonchai had not discussed the bride price yet.
A source close to Mr Boonchai confirmed the billionaire and his fifth wife Benja, a former actress, were already divorced.
The source said Mr Boonchai met Tak when she visited his museum in May this year and that he liked her because she was a "dutiful daughter" and he wanted to marry her.
Mr Boonchai had bought a 40-million-baht 30-rai bridal house and the engagement party would be held at the Museum of Contemporary Art, the source added.
Sararat Rumruangwong, one of Mr Boonchai's five former wives, said she and her former husband had never contacted each other in the 10 years since their separation.
Ms Sararat said she did not want to comment on Mr Boonchai's latest romantic relationship because it was none of her business.