Danny Fenster, who had been working for local outlet Frontier Myanmar for around a year, was arrested in May as he tried to leave the country to see his family.
He was sentenced to 11 years for incitement, unlawful association and breaching visa rules, his lawyer Than Zaw Aung told AFP.
His client had not decided whether he would appeal, he added.
Fenster, who has been held in Yangon’s Insein prison since he was detained, also faces charges of sedition and terrorism, which could see him jailed for life.
“Everyone at Frontier is disappointed and frustrated at this decision,” Frontier Myanmar said in a statement.
“We just want to see Danny released as soon as possible so he can go home to his family.”
The sentencing was based on evidence from the junta-appointed information ministry that showed at the time of his arrest Fenster had been working at local outlet, Myanmar Now, which had its licence revoked shortly after the coup, Than Zaw Aung said.
Fenster’s team were barred from cross-examining the ministry’s permanent secretary, he said, and the court did not consider tax evidence that Fenster had already left the outlet and was working for Frontier.
Crisis Group Myanmar senior advisor Richard Horsey described the sentence as “outrageous”.
“It sends a message not only to international journalists... but also Myanmar journalists that reporting factually on the situation is liable to get them many many years in prison,” he told AFP.
He noted US diplomats were working to get him released.
“It will be resolved through diplomatic channels and hopefully very quickly,” he said.
“But obviously this sentence is a big setback to US efforts.”
The sentencing comes days after former US diplomat and hostage negotiator Bill Richardson met junta chief Min Aung Hlaing in the capital Naypyidaw, handing the increasingly isolated junta some rare publicity.
Richardson, declining to give further details, said the US State Department asked him not to raise Fenster’s case during his visit.
Fenster is believed to have contracted Covid-19 during his detention, family members said during a conference call with American journalists in August.
He last spoke with US consular officials by phone on 31 October, State Department spokesman Ned Price said Monday.
Myanmar has been mired in chaos since a February coup, with the military trying to crush widespread democracy protests and stamp out dissent.
More than 1,200 people have been killed by security forces, according to a local monitoring group.
The press has also been squeezed as the junta tries to tighten control over the flow of information, throttling internet access and revoking the licences of local media outlets.
More than 100 journalists have been arrested since the putsch, according to Reporting ASEAN, a monitoring group.
It says 31 are still in detention.