She was suffering from an abdominal infection and "her condition continued to worsen", the Bureau of the Royal Household said in a statement, until she "passed away peacefully" on Thursday evening.
She will lie in state at the Grand Palace in Bangkok and her funeral would be held "with the highest honors according to royal tradition", it added.
Known in Thailand as "Princess Bha", the only child from King Maha Vajiralongkorn's first marriage had been in hospital since falling ill suddenly in December 2022.
The Bureau of the Royal Household said in May that her health condition had deteriorated and she was relying on medical devices to support her lung and kidney functions, as well as medication.
Bajrakitiyabha was the only child of King Vajiralongkorn's first marriage to Princess Soamsawali.
