A judge also found Officer Edward Nero not guilty of reckless endangerment and misconduct in office. As the verdict was read, Nero dropped his head down and his attorney placed a hand on his back. The courtroom was quiet. Nero stood up and hugged his attorney, and was visibly emotional.
The assault charge carried a maximum penalty of 10 years in prison and reckless endangerment carried a punishment of up to five years.
Gray died April 19, 2015, a week after his neck was broken in the back of a police transport van while he was handcuffed and shackled but left unrestrained by a seat belt.
Nero was one of six officers charged in the case. He waived his right to a jury trial, opting instead to argue his case before Circuit Judge Barry Williams. A jury trial was held for Officer William Porter late last year, and the panel could not reach a decision on manslaughter and other charges.
Gray's death set off more than a week of protests followed by looting, rioting and arson that prompted a citywide curfew. His name became a rallying cry in the growing national conversation about the treatment of black men by police officers.
Mayor Stephanie Rawlings-Blake said in a statement that Nero will still face an administrative review by the police department.
"We once again ask the citizens to be patient and to allow the entire process to come to a conclusion. In the case of any disturbance in the city, we are prepared to respond. We will protect our neighborhoods, our businesses and the people of our city," she said.
About a dozen protesters gathered outside the courthouse as the verdict was read.
Prosecutors said Nero unlawfully detained Gray and acted callously when he made a decision not to buckle Gray into a seat belt when he was loaded into the back of a transport vehicle.
Nero's attorney argued his client didn't arrest Gray and that it was the police van driver's responsibility to buckle in detainees. The defense also said the officers who responded that day acted responsibly, and called witnesses to bolster their argument that any reasonable officer in Nero's position would have made the same decisions.
The defense said the department's order requiring that all inmates be strapped in is more suggestion than rule because officers are expected to act with discretion based on the circumstances of each situation.
The other officers are set to each have separate trials over the summer and into the fall. Nero is white and Porter is black. Two of the other officers charged in the case are white and two are black.