Harrison, 49, was arrested on July 8 after his adopted Russian-born son, Chase, died of heatstroke after being left in a vehicle in the sun for nine hours in front of his father's workplace.
"The court does not find beyond a reasonable doubt that the elements of involuntary manslaughter have been met. For this reason, the defendant is found not guilty," Fairfax Circuit Court Judge R. Terrence Ney said.
The standard under Virginia law for involuntary manslaughter is "negligence so gross, wanton and culpable as to show a callous disregard for human life."
A guilty verdict would have carried a sentence of up to 10 years in prison.
"No prison term is going to cause more pain than that which he has already suffered," Judge Ney said. "The only true atonement here can only take place within his heart and soul," Ney said, "and he's very fortunate that he's been supported by such a strong and loving family, friends, co-workers and neighbors."
On the morning of July 8, Harrison was asked by his wife, Carol, to drop Chase off at the child daycare center on his way to work. On the way, he stopped off at a drycleaner's, and then, forgetting that his son was in the car, continued to work and headed straight into the office.
Carol usually took Chase to the center, according to court reports.
The child remained in the car for nine hours on a 90-degree day, according to earlier investigative reports.
The boy, born Dmitry Yakolev and renamed after his adoption, had been living in a children's home in northwest Russia's Pskov Region until the Harrisons adopted him just three months prior to his death. He was developmentally delayed and had not started talking.
Over the last 10 years in the United States there have been approximately 230 cases of parents locking their children in cars on a hot day.