News agency AFP noted in a profile of Ubolratana on Friday that her post accepting Thai Raksa Chart's nomination for prime minister candidate featured her wearing a traditional dress from that region.
The party thanked the 67-year-old princess for her efforts, and said it would abide by election regulations and "tradition and royal customs" under Thailand's constitutional monarchy.
King Vajiralongkorn, 66, issued his message late on Friday, saying his elder sister's candidacy was "inappropriate" and it was against the spirit of the constitution for royalty to be involved in politics.
The palace, however, said she is still a member of the ruling dynasty. Thailand also enforces strict laws banning criticism of the king or other royal family members.
Just eight years after ousting Thaksin, the military stepped in again to remove his sister's government.
Hours earlier, Princess Ubolratana defended her decision to run for office.
It started at around 9.15 am, when allies of former premier Thaksin Shinawatra - a tycoon who has lived in exile for more than a decade after repeated clashes with royalists - said former princess Ubolratana Rajakanya had agreed to stand as prime minister.
Ubolratana is also known as a long-time friend of the Shinawatra family, which has an influence on the upcoming election through its proxy political parties without fielding a direct family member this time.
Thai Raksa Chart's statement said the party loyally accepted the king's order and expressed deep gratitude to Ubolratana for her kindness toward the party.
"We are in new territory", said Kevin Hewison, an emeritus professor at the University of North Carolina at Chapel Hill who has written about Thai politics for decades.
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Rivalry between the Bangkok-centred, royalist elites and Thaksin and his rural-based supporters has brought street protests, military coups, and violent clashes over nearly 15 years.
Pheu Thai is also fielding candidates in the election, and Thai Raksa Chart was 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 seats, or to act as a back-up if the main party was disqualified.
She was virtually disowned by her father in 1972 when she married American Peter Jensen, who was a fellow student at the Massachusetts Institute of Technology.
But Thai Raksa Chart Party leader Preechapol Pongpanich told reporters at the Election Commission he was not anxious about challenges.
She said she wanted to exercise her rights as an ordinary citizen by offering her candidacy for prime minister.
The nomination pits the princess against current Prime Minister Prayuth Chan-ocha, the preferred candidate of the military, considered one of Thailand's most royalist institutions.
1975: She receives a master's degree in public health at the University of California, Los Angeles. Ubolratana falls into a gray area, since she is commonly called a princess and treated as such, despite losing the royal designations after her marriage. Like all the parties contesting the election, it had to submit its candidate list to the Election Commission on Friday.
"I would like to say once again that I want to see Thailand moving forward, being admirable and acceptable by global countries, want to see all Thais have rights, a chance, good living, happiness to all", she said, concluding with "#ILoveYou". She is referred to as "Tunkramom Ying" meaning "Daughter to the Queen Regent".
She was seen at the World Cup in Russian Federation smiling with Thaksin and his sister Yingluck - also a former prime minister - and has over the junta years given a number of coded nods on social media in favour of the Shinawatras.