Princess Bajrakitiyabha is widely seen in Thailand as the most plausible heir to the throne.
Princess Bajrakitiyabha, the eldest daughter of King Vajiralongkorn, collapsed while training her dogs north-east of Bangkok, the palace said.
The 44-year-old was taken to a nearby hospital, then taken by helicopter to Bangkok, where she is being treated.
The palace described her condition last night as "stable to a certain extent".
Medical bulletins from the royal palace in Thailand are typically vague and cryptic, and from the single statement issued about Princess Bajrakitiyabha, it is difficult to gauge how serious her condition is, says the BBC's Jonathan Head in Bangkok.
The statement says nothing about her state of health now. Some reports have suggested it is a lot more serious than stated.
The princess is the daughter of the king's first wife Princess Soamsawali, and his eldest child. She has been a part of her father's innermost circle since he succeeded King Bhumibol in 2016, and has been made a senior officer in the king's personal guard.
She is the most visibly accomplished of the inner royal circle, with post-graduate degrees in law from two US universities.
King Vajiralongkorn has not yet named an heir to the throne, but Princess Bajrakitiyabha is widely viewed as the most suitable successor.
Being one of the king's three children who have formal titles, she is eligible for the throne under a 1924 Palace Law of Succession.
She is a fitness enthusiast and also has a long record of advocating penal reform in Thailand.
She is especially vocal for female prisoners, of which Thailand has one of the world's highest populations.
She was Thailand's ambassador to Austria from 2012 to 2014.
-- Courtesy of BBC News