Princess Ubolratana Rajakanya Sirivadhana Barnavadi, 67, the elder sister of King Maha Vajiralongkorn, will run as a candidate for a party loyal to ousted premier Thaksin Shinawatra, the figure at the center political turbulence and rival street protests that have riven Thai society for years.
One of her leading opponents in the March 24 election will be Prime Minister Prayuth Chan-ocha, the leader of the ruling 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 commands the devotion of millions.
It was not immediately clear whether the nomination of Ubolratana, an avid social media user and former soap opera star who lived in California for years, had the approval of King Vajiralongkorn.