Cleveland Officer Michael Brelo, who climbed on to a car bonnet and fired repeatedly through the windscreen at unarmed black occupants, has been cleared of all charges.
The victims, Timothy Russell and Malissa Williams, died in 2012 in a barrage of bullets fired by 13 officers in a car chase.
Michael Brelo, 31, the only police officer to be charged, was cleared of voluntary manslaughter at the court in Cleveland.
The US has seen a series of police race-related killings, some of which have led to serious civil unrest.
Community and city leaders were braced for possible unrest over the latest ruling.
Some 30 protesters gathered outside the cordoned-off courthouse, some chanting “hands up, don’t shoot!” They were faced off by an equal number of sheriff’s deputies.
The deaths of Timothy Russell, 43, and Malissa Williams, 30, had sparked a department of justice inquiry that concluded Cleveland police had engaged in a pattern of excessive force and violation of civil rights.
Only Michael Brelo was charged because prosecutors said the car occupants were no longer a threat when he climbed on to the bonnet of their car and fired 15 shots through the windscreen in 7.4 seconds.
However, the judge said that as other officers had fired in a 137-shot barrage, he could not rule beyond reasonable doubt that Michael Brelo was responsible for the deaths.
Cuyahoga County Common Pleas Judge John O’Donnell also cleared Michael Brelo of the lesser count of felonious assault.
The judge said he would not “sacrifice” Michael Brelo if the evidence did not warrant conviction.
“Guilty or not guilty, the verdict should be no cause for a civilized society to celebrate or riot,” said the judge.
County prosecutor Timothy McGinty said he was “profoundly disappointed” with the verdict.
Timothy Russell’s sister, Michele, said Michael Brelo would not “dodge this just because he was acquitted. God will have the final say”.
Michael Brelo had faced up to 22 years in jail if convicted of voluntary manslaughter.
The incident occurred when the car, a Chevy Malibu, backfired while speeding past Cleveland police HQ, and officers thought a gun had been discharged.
Some 62 police cars were then involved in a 22-mile chase at speeds up to 100 mph.
Both Timothy Russell and Malissa Williams had previous convictions and a crack pipe was found in the car.
Michael Brelo’s lawyer, Patrick D’Angelo, said his client had risked his life during the chase and that the prosecution of the case was “vicious and unprofessional”.