A lawyer for Ekachai Hongkangwan said soldiers arrested Ekachai at his Bangkok home on Tuesday morning and indicated they would detain him outside the city, in Kanchanaburi province.
Ekachai commented on Facebook last week that he would wear red on October 26, the day Bhumibol will be cremated during an elaborate five-day funeral, and "do what people least expect."
Supporters of elected governments ousted by Thailand's military in 2006 and 2014 coups are known as "Red Shirts."
Ekachai previously served nearly three years in prison for violating Thailand's draconian lese majeste law by distributing copies of an Australian documentary on the Thai royal family.
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After he made the Facebook posting Friday, Ekachai became a target for Thai royalists who bombarded him with attacks on social media.
Thailand's monarchy is protected by a strict law that mandates prison sentences of up to 15 years for insulting the king and other senior royals. It has been used with increased frequency by the military government that seized power in the 2014 coup.