Prawet Prapanukul, 57, a human rights lawyer, is led Wednesday afternoon from a Bangkok court room and returned to custody.
Prawet appeared in a Bangkok court on Wednesday and was charged with 10 counts of royal insult and three counts of breaking Section 116 of Thailand's criminal code, the equivalent of sedition.
"Prawet faces 10 counts of breaking Article 112, so that works out to up to 150 years in prison if he is found guilty", Anon Nampha of Thai Lawyers for Human Rights told Reuters.
Thailand's military seized power from an elected civilian government in a May 2014 coup.
Global rights watchdog Human Rights Watch earlier this week demanded the military junta reveal the whereabouts of the 57-year-old lawyer, who reportedly went missing after his home was raided last Saturday. Each count carries up to 15 years in jail and the section has been used with particular ferocity in recent years.
A spokesman for the military government said he was unable to comment on the case.
Rights groups expressed shock at the latest charge.
"Imprisonment is never a proportionate penalty for the exercise of free expression, let alone the unthinkable possibility of 150 years", Kingsley Abbott, from the International Commission of Jurists, was quoted telling the AFP.
Ekachai said Prawet was refusing to answer questions and insisted he would only give statements to the court.
Three of the other five people arrested were charged under the computer law for sharing on social media a post by a prominent exiled critic of the government, historian Somsak Jeamteerasakul.
Prawet is in pre-trial detention at a Bangkok remand prison after being held incommunicado at the 11th Army Circle base in Bangkok, a facility the military uses as a temporary prison.
HRW urged the military to observe due process.
"Even though lese majeste is a serious criminal offense in Thailand, authorities have no justification to breach due process and fair trial standards", Sunai Phasuk, Thailand researcher at Human Rights Watch, told the Reuters news agency.
Laurent Meillan, the agency's acting regional representative, said in a statement that he was very concerned by the sharp increase in the use of the lese majeste law after the 2014 coup, and that United Nations human rights experts have repeatedly stated that the implementation of the law violates the fundamental rights of freedom of expression and opinion.
Thailand's lese majeste law is the world's harshest and routinely draws criticism from rights groups, which say it is open to politically motivated abuse.
King Maha Vajiralongkorn, 64, ascended the throne in December following the death of his father, King Bhumibol Adulyadej, in October.