Hok Chun Anthony Kwan, from Minneapolis, Minnesota, was detained when he was about to board a plane on August 23 after covering the aftermath of a deadly bomb explosion at a shrine in Bangkok.
Kwan, a Hong Kong and Canadian citizen, works for the Hong Kong-based Initium media group.
His lawyer, Pawinee Chumsri, said he pleaded not guilty to the charge of weapons possession after being indicted by the court, which set a pre-trial hearing for November 16.
Pawinee said Kwan did not intend to violate Thai law. "Kwan believes he is not guilty. He only brought the jacket to protect himself," she said.
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Under the Arms Control Act, a license is needed to possess body armor, which is considered a weapon. Violations are punishable by up to five years in jail. The law has rarely if ever been enforced for journalists covering the country's sometimes-violent political turmoil over the past nine years.
Many large news organizations require their staff to wear protective gear in dangerous situations.
The Foreign Correspondents' Club of Thailand expressed disappointment over the filing of formal charges against Kwan.
Pawinee said Kwan is free on bail and will travel back to Hong Kong tomorrow.