Former royal bodyguard and army nurse Sineenat Wongvajirapakdi was dismissed from the rank of Chao Khun Phra, or noble consort, and accused of "disloyalty to the king".
Her whereabouts are unknown but the announcement has set social media alight with speculation and intrigue sparked by the rare glimpse behind cloistered palace walls.
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But it wasn't the first time King Maha Vajiralongkorn has made his powerful presence felt since ascending the throne in 2016.
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The palace said Sineenat had not obeyed the king, had created conflict in the palace by going above her authority, and had acted "against the appointment of the Queen (Suthida)... for her own ambitions".
Her actions had also undermined the monarchy, it said.
Known as Koi, Sineenat is a qualified pilot and was the first woman to receive the consort title in nearly a century.
After she obtained it, the palace released images of a short-haired Sineenat in combat fatigues shooting weapons, flying a jet and preparing to parachute from a plane, as well as holding the king's hand.
It was a vivid contrast to the swift dismissal, which has gripped Thai Twitter users.
#SaveKoi was one of the trending hashtags, while one user said they felt "sorry" for Sineenat.
Thailand has a harsh royal defamation law that means media must self-censor when reporting on the palace.
Sineenat's rapid fall from favour is not the only time the Thai public has been given a glimpse into the king's dramatic personal life.
Several relatives of his third wife Srirasmi were arrested as part of a police corruption probe and charged with lese majeste.
Vajiralongkorn divorced her and she lost her royal titles, and pictures later circulated of her with a shaven head.
The palace has also stepped up royal security under Vajiralongkorn, adding 1,600 police officers.
Two new army units were later put under the monarch's direct control in October by royal decree, only a few months after the May coronation.
It is unclear how many soldiers are in each regiment, but experts say a single division holds up to 5,000 troops.
The troop transfer saw pushback from the youth-propelled Future Forward Party in parliament who questioned the "urgency" of such a move -- an unprecedented political objection to a royal command.
But the decree still passed with an overwhelming majority.
King Vajiralongkorn inherited one of the world's richest monarchies from his beloved father Bhumibol Adulyadej, who died in October 2016.
Among his successor's earliest moves was to assert full, personal control over the Crown Property Bureau (CPB), which has billions in assets in banks, companies and prime real estate.
The CPB committee was previously headed by the finance minister in an arrangement that gave a sheen of public oversight of a trust some experts estimate to be worth $30-$60 billion.
The CPB's full assets are privately held and remain a closely guarded secret.
Discipline, protocol and loyalty are virtues state officials say the king insists upon, especially among royal guards.
Vajiralongkorn reminded members of his beefed-up detail in April that their duty was to "preserve the nation, country, and monarchy".
The army introduced a new salute under Vajiralongkorn in which soldiers must puff out their chests and jerk their heads to the side.
And the country's police force has adopted a uniform crew-cut said to be at the king's request.
Any major legal changes in Thailand require the king's signature.
But King Vajiralongkorn ordered rewrites to part of the country's new constitution, including a vaguely worded clause that says any unforeseen issues should be handled based on "tradition".
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They also scrap the need for the king, who spends much of his time in Germany, to appoint a regent when abroad.
While Thailand's constitutional monarchy is nominally above politics, Vajiralongkorn has taken actions that many see as smudging the line.
Before the March 24 election he torpedoed a bid by his older sister to become prime minister at the head of an anti-junta party, calling it "highly inappropriate".
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