A Queen's Bench justice ruled that Lethbridge constable David Easter acted in self-defence and used reasonable force when he punched a man in the face.
Court heard Donovan Girard was drunk outside a downtown bar and resisted when Easter tried to arrest him in May 2012.
Girard went down and hit his head on the sidewalk.
Easter testified during his trial that the only reason he hit Girard was because he believed Girard was going to strike him.
Easter has been a police officer for the last 15 years.