PC Colin Leask, 29, of the Grampian force, appeared for sentencing at Aberdeen Sheriff Court after earlier being found guilty of assaulting 19-year-old Kevin McGeachy, who was being held in a cell at police headquarters in Aberdeen last July.
Leask, a martial arts expert since the age of 12, had denied the assault charge. He was released on bail pending appeal.
The court had been told at the earlier hearing how 18-stone Leask kicked Mr McGeachy between the legs before unleashing a flurry of blows to his head and throwing him to the ground where he split his head.
Mr McGeachy had been under the influence of drink and drugs at the time and had been taken into custody for a strip search following a disturbance, in Aberdeen city centre, the trial was told.
Other officers in the cell said Leask's attack was totally unprovoked.
Leask claimed he only used a martial arts self-defence technique when he thought Mr McGeachy was about to kick and hit him.
Mr Shepherd told the court his client's eight-year career in the police force was "now in ruins".
Jailing Leask, Sheriff Graeme Warner said society would "return to the old days of South Africa if policemen were allowed to beat up people they didn't care for very much".
He would be failing in his duty if he did not send Leask to prison. It was totally unacceptable for police officers to assault people in custody.
During the trial, the court was told that Leask was a black belt in karate and expert in various forms of martial arts. He had competed in martial arts tournaments at home and abroad.Reuse content