The nomination of Princess Ubolratana Rajakanya Sirivadhana Barnavadi, 67, the elder sister of King Maha Vajiralongkorn, promised to upend Thailand's already turbulent politics because it breaks the long-standing tradition of Thai royalty staying out of politics.
Princess Ubolratana will run as a candidate for a party loyal to ousted premier Thaksin Shinawatra.
One of her leading opponents will be Prayuth Chan-ocha, the leader of Thailand's military junta, who also announced his candidacy on Friday.
Thailand has been a constitutional monarchy since 1932 but the royal family has wielded great influence and commanded the devotion of millions.
The election shapes as a battle between Thaksin's populists and their allies and the royalist-military establishment.
However, the nomination of a member of the royal family by the pro-Thaksin Thai Raksa Chart party could change that dynamic.
Thai Raksa Chart is an offshoot of the Pheu Thai Party, formed by Thaksin loyalists and the core leadership of the United Front for Democracy Against Dictatorship (UDD), or "red shirts" group, as a strategy to help Pheu Thai win votes.
The simmering conflict between the Bangkok-centered elites and the more rural-based populists has resulted in street protests, military coups, and violent clashes over almost 15 years.
"The party has nominated the princess as its sole candidate," Thai Raksa Chart Party leader Preechapol Pongpanich told reporters after registering his party's candidate at the Election Commission.
"She is knowledgeable and is highly suitable. I believe there will be no legal problems in terms of her qualification, but we have to wait for the Election Commission to endorse her candidacy," he said.
The Election Commission is required to endorse all candidates by next Friday.
Reuters could not independently confirm whether Princess Ubolratana's nomination had the approval of the palace.
Prayuth accepted his nomination from the Palang Pracharat Party in an official statement.
"I am not aiming to extending my power but I am doing this for the benefit for the country and the people," he said. There was no mention of the princess's nomination in Prayuth's statement.
Princess Ubolratana, the oldest daughter of King Bhumibol, was born in Lausanne in 1951.
She studied mathematics and bio-chemistry at the Massachusetts Institute of Technology (MIT) and earned a master degree in public health from the University of California at Los Angeles.
Princess Ubolratana relinquished her royal titles in 1972 when she married an American, her fellow MIT student Peter Jensen.
She lived in the United States for more than 26 years before they divorced in 1998.
She returned permanently to Thailand in 2001, performing royal duties but never regaining her full royal titles.
She is referred to as "Tunkramom Ying", which means "Daughter to the Queen Regent", and is treated by officials as a member of the royal family.
Princess Ubolratana is known for her "To be Number One" philanthropy campaign, which aims to help young people stay away from drugs, as well as starring in several soap operas and movies.
An avid social media user, she recently posted videos eating street food and another complaining about pollution in Bangkok.