The ruling marks the first time a journalist has been convicted under Myanmar's new government, run by former political prisoner Aung San Suu Kyi.
The reporter, Nay Myo Lin, who works for the BBC Myanmar language service, plans to appeal the decision.
His lawyer, Thein Than Oo, called the sentence "unfair," saying that his client had not intentionally hurt the policeman and was trying to help a fellow citizen.
The incident occurred in March 2015 when Nay Myo Lin was covering a protest near Yangon, Myanmar's biggest city, against an education bill that students said was aimed at stifling academic freedom.
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A policeman knocked a protester off his motorbike and Nay Myo Lin tried to help the man, leading to a scuffle with the officer, Thein Than Oo said.
Nay Myo Lin was convicted of assaulting a public servant and sentenced yesterday by a court in the city of Mandalay, where he is based.
Suu Kyi's party, the National League for Democracy, won a landslide victory in November elections, ushering in the country's first civilian government after 54 years of direct and indirect military rule.
Both the police and the judiciary are overseen by Myanmar's Home Ministry, which remains under control of the military.