Fenster, 37, managing editor of online magazine Frontier Myanmar, was arrested while trying to leave the country in May and has since been held in Yangon's notorious Insein prison.
The sentence was confirmed to Reuters by lawyer Than Zaw Aung, who provided no further details.
It was handed down for incitement and breaches of immigration laws and unlawful associations laws, his employer, Frontier Myanmar, said.
New, more serious charges of sedition and violations of the terrorism act were added earlier this week, which are punishable each by a maximum 20 years in prison.
"There is absolutely no basis to convict Danny of these charges," said Thomas Kean, Frontier Myanmar's editor-in-chief.
"Everyone at Frontier is disappointed and frustrated at this decision. We just want to see Danny released as soon as possible so he can go home to his family."
The United States has been pushing for Fenster's release. The US embassy in Myanmar did not immediately respond to a request for comment.
A spokesman for the ruling military council could not immediately be reached.
Myanmar has been in chaos since a military coup in February that ousted ainst the elected government of Aung San Suu Kyi.
More than 1,200 civilians have been killed and scores of journalists are among thousands who have been detained amid a backlash to military rule, according to activists cited by the United Nations.
In remarks prior to his sentencing, which was not publically announced, the US State Department said: "The profoundly unjust nature of Danny’s detention is plain for all the world to see. The regime should take the prudent step of releasing him now."