Thai Princess Bajrakitiyabha Mahidol, the king's eldest daughter, has died aged 47, the royal palace announced on Friday, more than three years after she was hospitalized following a sudden illness.
She was suffering from an abdominal infection and "her condition continued to worsen" until she "passed away peacefully" on Thursday evening, the Bureau of the Royal Household said in a statement.
The late princess will lie in state at the Grand Palace in Bangkok and her funeral will be held "with the highest honors according to royal tradition", it said.
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, AFP reported.
The Bureau of the Royal Household said in May that her 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 marriage to Princess Soamsawali.
A trained prosecutor and diplomat, Bajrakitiyabha was educated in Britain, Thailand and the United States, earning a law degree from Cornell University and serving for a time as Thailand's ambassador to Austria.
The princess told a campus audience about her professional background during a 2012 visit to the US law school. "I ask myself now, what am I exactly? A prosecutor? A criminal lawyer? A diplomat? The answer is everything all together. I say I am a hybrid," she said, according to the Cornell Chronicle.
She also held several positions with the United Nations and became an advocate for women's rights, including improved conditions for women in prison.
"She was loved, respected and admired by people throughout the kingdom. She was kind, talented and of exemplary conduct," Prime Minister Anutin Charnvirakul said in a televised address on Friday.
"She dedicated her life to promoting justice, equality, human dignity and rights in society. I invite all Thai people to join in mourning her passing and to follow her example as an inspiration in serving the king and the monarchy."
As princess, Bajrakitiyabha held an important ceremonial role in Thai society -- where the royal family sits at the apex.
She was seen as close to her father, and was appointed to a senior role in his bodyguard command a year before her hospitalization.
A royal procession carrying her body is scheduled to move on Saturday afternoon from the hospital to the Grand Palace in Bangkok, where traditional royal rites will be conducted in the Piman Rattaya Hall.
The government has invited members of the public to pay their respects and make merit, and official buildings will lower flags to half-mast for 15 days.
"I feel sad, very sad, because she did so much good for the Thai people," Namooy Jaengklaykom, a 53-year-old tailor, told AFP outside the palace.
"It happened too soon."
Mourners gathered on Friday to pay their respects at Chulalongkorn Hospital, where Bajrakitiyabha had been receiving treatment, some clutching portraits of the late princess.
"When I heard the announcement, I was very sad," said Thanyaporn Arammekha, a 66-year-old retiree whose eyes were swollen from crying.
"I love the monarchy because my parents divorced when I was very young. Rama IX was like a father figure to me," she said, referring to the former king.
She said she had visited the hospital regularly while the princess was receiving treatment and had rushed there as soon as she had heard the news.
Kanokpan Chantarapetch, 67, a retired provincial official, also came to pay her respects.
"I can't really speak. I'm overwhelmed," she told AFP through tears.
"I have loved Princess Bha since she was very young," she said, adding that "as a former government worker, I understand how much the royal family has done for the country".
Queen Sirikit, the mother of the king, died in October at the age of 93.
The 73-year-old king, who has seven children from four marriages, has not announced his chosen heir, although succession rules favor men.
Strict rules govern what can and cannot be said about the Thai royal family, who are protected from criticism by lese-majeste laws that carry prison sentences of up to 15 years per charge.
