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All the defendants faced one joint count of causing grievous bodily harm with intent, but in the end the city's court convicted them with a lesser charge of assault occasioning actual bodily harm.
The convicted officers were sentenced to two years in prison each.
One of the defendants, policeman Chan Siu-tan, 31, was also found guilty of common assault for slapping social worker Ken Tsang Kin-chiu twice during an interview, at a police station in Hong Kong's Central district.
Chan was sentenced to one month in prison for the assault to be served concurrently with his two year sentence.
The verdict was handed down on Tuesday, while sentencing took place on Friday.
Outside the District Court, protestors who supported the policemen shouted slogans and faced off against pro-democracy supporters, who demanded justice.
Beating captured on video
Tsang, a member of Hong Kong's pro-democracy Civic Party, was beaten by the police officers on the night of October 15, 2014 during the large pro-democracy protests that were dubbed the "Umbrella Movement."
His beating was captured on video and caused widespread condemnation of police behavior in the city after it was aired on local TV station TVB.
Tsang, who was unarmed at the time, was hospitalized after the incident and sustained serious bruising.
In a separate case, Tsang was charged with assaulting police officers and resisting arrest on the same night he himself was beaten. He was found guilty in May 2016 and jailed for five weeks.
CNN's Mark Lu contributed to this report.