"He was arrested on Sunday at a hospital. He posted a false infographic on social media," junta spokesman Colonel Winthai Suvaree told AFP.
The 25-year-old, identified by Thai media as Thanet Anantawong, shared the infographic about Rajabhakti Park, a multi-million-dollar park built by the military under construction contracts allegedly riddled with kickbacks.
The generals partially justified their coup last year as necessary to curb endemic corruption among civilian politicians and protect the royal family.
"He was charged by police for violating computer crime act, defamation, and sedition," Winthai said, declining to specify where the man was being held except that he was "in the control of authorities".
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The junta leadership, including Prime Minister General Prayut Chan-O-Cha, has insisted there was no graft related to the park. But the accusations have still swirled in local media, transfixing a country where dissent has become rare.
Today a military court in Bangkok approved Thanakorn's detention after he had been held incommunicado at an unspecified military base for several days, a court official told AFP on condition of anonymity.
Under Thai law anyone convicted of insulting the revered but ailing 88-year-old King Bhumibol Adulyadej, or the queen, heir or regent can face up to 15 years in jail on each count.