Caesar Goodson was the third of six officers charged over the death of the 25-year-old, which sparked riots in Maryland's largest city last year. No guilty verdicts have been handed down in the three trials.
Because the state failed to meet its burden of proof in any of the seven charges against Goodson, "the verdict on all counts is not guilty," said Judge Barry Williams, who presided over the case.
Williams had challenged state prosecutors' charge that Goodson had deliberately given Gray a "rough ride" in the back of the van.
In delivering his verdict he said there was no evidence of excessive speed, or malicious "actions or intent" by Goodson, nor did the state show that the defendant had a specific reason to target Gray through his actions.
Baltimore Mayor Stephanie Rawlings-Blake, who is black, said the officer would now face an administrative review and appealed for patience in allowing the process to run its course.
"I know the citizens of Baltimore will continue to respect the judicial process and the ruling of the court," she said.
Judge Williams dwelled on the dispute between medical experts over exactly when Gray's critical injury occurred, saying his injuries were internal thereby making it impossible to determine where and when they were sustained.
None of Officer Goodson's conduct rose to the level of criminal activity, he said.
"Simple carelessness is insufficient to establish the defendant's guilt," Williams said.
The two other officers tried in the case -- William Porter, whose trial ended in a hung jury in December, and Edward Nero, who was acquitted of all charges -- were reportedly in the court room and seen hugging and shaking hands after the verdict.
