A judge also found Officer Edward Nero not guilty of reckless endangerment and misconduct in office. 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.
Officer William Porter's manslaughter trial ended in a hung jury in December.
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.