There wasn't enough evidence to prove beyond a reasonable doubt that Const. Glenn Weddell shoved and hit Dorian Barton on June 26, 2010, the judge said.
Barton's shoulder was broken that day and he alleged a police officer hit him with a riot shield, knocking him to the ground, and that he was hit several more times, possibly with a baton.
But Barton "was unable to say precisely where he was hit, by whom, with what, or what injury the impact caused, if any," Justice Gregory Ellies said in his ruling.
"He was also unable to say who struck the initial blow," a mystery even photos and video of the incident couldn't clear up, Ellies said.
While an eyewitness, Andrew Wallace, testified he saw Weddell strike Barton, his credibility was "tainted" by "the animosity he displayed towards the police" while on the stand, the judge said.
Weddell — who had pleaded not guilty to assault causing bodily harm and assault with a weapon — testified the only contact he had with Barton that day was to help him off the ground.
The officer expressed his relief outside the courthouse, saying living with the accusations had been "quite difficult" for him and his family.
Now that the cloud of suspicion has been lifted, "I just want to get back to doing what I love the most — policing the city," he said.
The case "demonstrates the difficulty that we had in policing the G20," and the lingering animosity toward law enforcement, said Mike McCormack, president of the Toronto Police Association.
"We saw some credibility issues ... and some bias toward the police, I'm sure we're going to see a lot more of that in other trials that are coming up (in relation to the G20)," he said.
That prosecutors relied on testimony from a single witness rather than calling on the other officers who were present at the time shows they didn't want to get to the bottom of the case, Barton's father said in a phone interview Friday.
"If that's all the Crown is going to present is one witness without anybody else, we felt it was doomed right there," Ted Burley said, adding he and his son were surprised that so many questions were allowed to remain unanswered.
"If it wasn't Weddell, who was it? That's the question that should've been asked of multiple officers," he said.
He dismissed the suggestion that police have been unfairly portrayed following the meeting of world leaders, which saw hundreds arrested only to have the charges against them dropped shortly afterward.
"There's only been two (police officers) facing prosecution" out of all those who took part in the mass arrests, he said. "So how they're all being victimized is beyond me."
The other officer facing charges stemming from the G20 protests, Const. Babak Andalib-Goortani, is set to go to trial next week on two counts of assault with a weapon.
In delivering his verdict Friday, Ellies said he didn't believe Wallace purposely mislead the court or meant to falsely accuse the officer.
"But I do believe that his ability to perceive the events at the time they occurred and his ability to recall and to recount them here has been tainted by the experience he had when he was wrongfully accused of sexual assault more than 15 years ago," he said.
The judge also said he had "concerns" Weddell's recollection of the incident might have been affected by photos and videos presented as evidence.
The officer testified he didn't remember meeting Barton and had to reconstruct what happened that day by viewing the images and footage.
Still, Ellies said, "I believe him when he says he did not strike Mr. Barton with his shield and knock him to the ground. That is something that I think he would remember."
Weddell was part of the public order unit and had been at several other scenes that day, at which people using so-called black bloc tactics broke away from peaceful demonstrations and began vandalizing parts of the city and clashing with police.
Later that day he was called to the Ontario legislature, which had been designated as a zone for peaceful protest. Barton had gone to that area with a friend to scope out the scene.
Video of Barton's arrest begins with the now 32-year-old Toronto man on the ground and shows two officers helping him up, then one of them gives Barton a shove and Barton appears to trip over a curb and falls to the ground.
None of the videos or photos presented at the judge-only trial showed the blows, and Ellies said he couldn't be sure exactly what happened.