Thailand's Royal Household Bureau says Princess Galyani Vadhana died Wednesday at the age of 84.
The elder sister of Thailand's King Bhumibol Adulyadej had been hospitalized since June of last year and was undergoing treatment for abdominal cancer at Bangkok's Siriraj Hospital.
Officials said in a statement that the princess's condition had been deteriorating gradually and that she died in the early morning hours.
The palace announced a 100-day mourning period for royal officials and said the princess's funeral would be held at Bangkok's Grand Palace.
The government announced a 15-day mourning period for state workers. Politicians negotiating the formation of a coalition government also postponed their activities today.
The U.S. embassy in Bangkok sent a condolence message to the royal family, praising Princess Galyani's charitable work.
The princess was born May 6, 1923, in London, England. She was well-known for her interest in the arts and for chairing several charities during her life.
She also taught French at Thai universities and later headed the Association of French Teachers in Thailand to help promote the language.
The princess's death comes after the 80-year-old king was hospitalized for three weeks last year, suffering symptoms of a stroke.
Some information for this report was provided by AFP, AP and Reuters.