Judge Louise Botham said the officer, Constable Babak Andalib-Goortani, was "not entitled to use unlimited force to effect an arrest," the Canadian Broadcasting Corp. reported. Thursday.
Government prosecutors had argued the officer had "lashed out" at protester Adam Nobody during a demonstration June 26, 2010, in front of the Ontario legislature.
Nobody was tackled by police. Prosecutors said Andalib-Goortani hit Nobody with a baton after he was already on the ground.
Andalib-Goortani said he administered the blow because Nobody was resisting.