The Crown had argued that Const. Babak Andalib-Goortani was overwhelmed by the chaos and "lashed out" at protester Adam Nobody, hitting him with a baton after the man had been wrestled to the ground.
Judge Louise Botham told the court "a police officer is not entitled to use unlimited force to effect an arrest."
Botham said Andalib-Goortani's explanation that he was responding to Nobody's resistance is nothing more than an after-the-fact attempt to justify his blows.
Nobody applauded when the verdict was read in the courtroom and told reporters outside he was "elated" about the decision.
Another cop in the courtroom let out a loud sigh. The protester was singled out for arrest at a demonstration on June 26, 2010 at the Ontario legislature. He was tackled as he ran from police.
In May, another officer, Const. Glenn Weddell was found not guilty of assault stemming from the G20 protests.