An official announcement said Sineenat Wongvajirapakdi was "ambitious" and had "attempted to elevate herself to the status of the queen".
"The royal consort's behaviours were considered disrespectful," it said.
Sineenat was the first person to be awarded the title of Royal Noble Consort in nearly a century.
She was appointed in July, just two months after the king's marriage to Queen Suthida, his fourth wife.
Sineenat, who was a major-general, is a trained pilot, nurse and bodyguard.
In August, official photos were released of her in combat fatigues and piloting a jet, alongside an announcement that the king had "ordered the creation of a royal biography" for her.
Queen Suthida - a 41-year-old former flight attendant and deputy head of his bodyguard unit - is King Vajiralongkorn's long-term partner and has been seen with him in public for many years.
He ascended the throne after the death of his father in 2016.
BBC
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