Prayuth Chan-ocha (pictured in 2020) has been temporarily suspended as a court determines if he's been in power longer than constitutionally allowed
Opposition parties have filed a case arguing that Mr Prayuth - in charge since 2014 - has overstayed his tenure.
Thailand's constitution limits prime ministers to eight years in office.
The ex-army chief first seized power in the 2014 military coup and then retained office in 2019 under a heavily restricted election.
However, in recent years he has been facing growing opposition, and backlash within his own coalition.
So far this year, he has survived multiple no-confidence votes against him.
Opponents and activists have argued Mr Prayuth's term began when he was junta leader. As military leader, he seized power in the May 2014 coup and then appointed himself prime minister of the new military government in August 2014.
That means his term should end this week, his critics say.
However his supporters say his term only began in 2017 - when a new constitution came into force - or even after a general election in 2019 that saw him retain power.
Under those terms, he could technically continue serving until 2027 - if he wins an upcoming general election.
On Wednesday, Thailand's Constitutional Court agreed to hear the case from the political opposition. The bench of judges ruled five to four in support of his immediate suspension.
It's unclear when the court will deliver its ruling on Mr Prayath's case.
Protesters had earlier gathered outside parliament buildings in the capital, Bangkok, demanding his resignation.
The move means Deputy Prime Minister Prawit Wongsuwan, 77, will become the interim prime minister, according to the cabinet line of succession.
Thailand is due to hold a vote before May next year, although the deputy PM had previously flagged it could be as early as November.
-- Courtesy of BBC News