Taweesak Kerdpoka was arrested on Sunday morning alongside four members of the New Democracy Movement.
A Thai journalist and four activists were charged Monday under a draconian law banning criticism of the junta's new constitution which will be voted on in a referendum next month.
Taweesak Kerdpoka, a reporter with the news outlet Prachatai, was arrested on Sunday morning alongside four members of the New Democracy Movement, one of the few activist groups that dare to challenge the military since generals seized power two years ago.
"Their actions were violating the referendum bill article 61 clause 2," police colonel Amnuay Pongsawat, from Ban Pong district in central Ratchaburi province, told AFP.
He did not elaborate on how their actions had broken the law, but the men face up to ten years in prison if convicted.
The clause outlaws critical discussion of the junta's new draft constitution, which if passed will become Thailand's 20th in less than a century.
Prachatai editor Chiranuch Premchaiporn said Taweesak was travelling in the same car as the activists to report on their activities.
Police stopped the car and found documents that they deemed were in breach the referendum law.
"He is a reporter that covers human and environmental rights," she told AFP. "He was just doing his job."
Thais will vote on the the new charter on August 7, the first return to the ballot box since the 2014 coup.
The junta says the document is the antidote to Thailand's caustic political divide.
But politicians on both sides of the divide have dismissed it as an attempt to further entrench the military's hold on power through an appointed senate.
Video posted online showed the five shackled men making their way to court on Monday morning as supporters handed them roses.
The activists could be heard shouting: "Voting 'no' is our right, it is not against the law".
A message from Taweesak on his Facebook page read: "Being arrested for referendum campaigning is bad, but what should we call being arrested for reporting on the referendum campaign?"
Prachatai has a history of investigative journalism that frequently riles Bangkok's ultra-nationalist establishment.
Last year its editor Chiranuch lost an appeal against a conviction under the country's notorious royal defamation law for failing to speedily remove reader comments deemed critical of the monarchy.
Since the military's takeover Thailand has undergone a major rights crackdown, with scores of activists jailed and skyrocketing lese majeste convictions.
The kingdom has been handicapped by more than a decade of political deadlock and violence including two military coups.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)