Princess Bejaratana Rajasuda Sirisobhabannavadi of Thailand (Thai: สมเด็จพระเจ้าภคินีเธอ เจ้าฟ้าเพชรรัตนราชสุดา สิริโสภาพัณณวดี) (born 24 November 1925) is the only daughter of late HM King Vajiravudh of Thailand. She is a first cousin of current HM King Bhumibol Adulyadej. She is officially styled Her Royal Highness Princess Bejaratana.

Princess Bejaratana (pronounced Pet-Cha-Rat) was born on 24 November 1925, the only child of HM King Vajiravudh (Rama VI) and HRH Princess Suvadhana. After having seen his only child for a day, the King died, on 25 November 1925. Her uncle, HM King Prajadhipok (Rama VII), performed the naming ceremony for the princess on 30 December.

The princess and her mother moved to Suan Hongsa Villa at Dusit Palace where she received her education from a private tutor. They then moved in with HM Queen Sri Savarindira (the Queen grandmother) during the war, and the princess attended Rajani School until the age of 12. The princess and her mother moved to England where she received further education as well as medication for her illness. They first stayed at Fairhill Villa in Surrey, before finally moving to Brighton.

In November 1957, mother and daughter moved back to Thailand. They bought a piece of land on Sukhumvit Road Soi 38, and build the Ruenruedi Villa Palace, which is still the home of the princess. Once the princess had settled into her new life in Thailand, she proceeded to undertake her Royal Duties representing the Thai Royal Family. Her special interests have been in connection to education, public health, Buddhism, the soldiers and police stationed at Thailand 's borders, and general public welfare.

Princess Bejaratana is known privately to be a very gifted individual, especially with numbers. She has the ability to calculate which day of the week for any dates presented to her, promptly and without any hesitation, as well as remembering the birthdays of all individuals who have been presented to her.

Due to her age and health she has lately cut down much of her Royal Duties, but once in a while, she still does some work that is connected to her late father.
thumb|right|230px|Royal cypher of Princess Bejaratana Rajasuda