The national carrier will move from Terminal 3 to Zone A of Terminal 2, known as the Queen’s Terminal in honour of Her Majesty Queen Elizabeth II, said Petchpring Sarasin, vice-president for corporate public relations.
Earlier, the airline planned to move this month but the terminal had to retest its bagage system.
The airline is one of 23 Star Alliance airlines operating flights to and from Heathrow that will make Terminal 2 their new home.
Heathrow’s Terminal 2 opened on June 4 this year.
THAI operates two daily flights from Bangkok to London.