Adrian Law, 45, was working as a detention officer at Barnsley police station when he put his arm through the hatch of a cell door and threw a cup full of extremely hot water over Abdul Aziz Alfadley, 26.
Leeds Crown Court heard that Mr Alfadley suffered serious injuries to his lower abdomen and genitals.
After more than nine hours of deliberations, a jury found Law guilty of causing grievous bodily harm. He was cleared of the more serious offence of causing grievous bodily harm with intent.
During his trial, the jury of seven men and five women, heard how Mr Alfadley was taken to the station on May 30 last year after he was arrested in Barnsley town centre for a public order offence. He was agitated and disruptive at the station and officers decided to strip search him, because they feared he was on drugs, before he was taken to a cell.
He was left wearing just his boxer shorts and was heard shouting and banging on the cell door as he lay on his back. The court heard how Law went to a kitchen and filled a cup from the hot water boiler and returned to the cell and threw it over the prisoner through a hatch in the cell door. Jurors were shown CCTV footage of the detention officer putting the cup through the hatch.
The judge, Mr Justice Spencer, said he accepted that Mr Alfadley was a very disruptive prisoner and that the job of a detention officer was very difficult.
But the judge told Law: “None of that is any excuse for what you did to Mr Alfadley. He was entitled to just the same standard of care and protection as any other detainee. If anything, his agitated state made him all the more vulnerable. It was a wicked thing you did.”
Law, of Cromwell Court, Goldthorpe, near Barnsley, denied the charge of causing grievous bodily harm with intent and the alternative of causing grievous bodily harm.
South Yorkshire Police Deputy Chief Constable Bob Dyson said: “The actions of Adrian Law on that day fell far below the standards that we expect from our staff in the custody suites, who owe a duty of care to those being detained. Mr Law let down the public and let down his colleagues. On behalf of the force, I apologise to Mr Abdul Alfadley for the injuries he received.”