The court, in a 25-minute reading of the verdict on Friday, said Prayuth’s tenure as prime minister should be counted from 2017, when a new constitution was promulgated.
The decision will be a boost for Prayuth, a staunch royalist whose premiership has been beset by attempts to unseat him, including four house censure motions, a conflict of interest case and major protests challenging his leadership and the monarchy.
Prayuth, 68, had been suspended from office while the court deliberated the case.
The Pheu Thai party and a government spokesperson did not immediately respond to separate requests for comment on Friday’s ruling.
Prayuth’s critics had argued that his time in office should be calculated from 2014, when he took power as army commander in the aftermath of a coup that removed Thailand’s elected Pheu Thai party government.
Supporters of Prayuth had maintained that his term in office should be calculated from at least 2017, or from when Prayuth took office after his election as a civilian prime minister in 2019.
In a surprise move, Prayuth was suspended as prime minister in August by the Constitutional Court, which had accepted a petition from Thailand’s political opposition calling for it to rule on whether the premier had exhausted his time in office.
Deputy Prime Minister Prawit Wongsuwan has served as Thailand’s caretaker premier since Prayuth’s suspension.
Prawit and Prayuth are former army commanders and were comrades in arms for decades. Prawit is widely expected to become prime minister if the court rules against Prayuth.