Sergeant Reggie Martinez, 24, was initially charged with involuntary manslaughter in the 3 January drowning of 19-year-old Zaidun Hassun near Samarra.
Martinez received a "non-judicial judgement" in the case, meaning he will not face criminal proceedings or jail time.
Defence attorneys have disputed the circumstances of Hassun's death after he and his cousin were picked up for being out after curfew. They said investigators bungled the case by not exhuming the body and doing an autopsy.
Investigators acknowledged they never saw the body, saying they relied on the word of relatives and a video made by Hassun's family that shows a corpse in a coffin.
A similar judgment was given to another soldier, Specialist Terry Bowman, 21, who was accused of assault for allegedly forcing a second man into the river. That man survived.
Details of the judgments were not disclosed, army officials said.
An army investigator testifying in Bowman's case said the soldier had said he was ordered to do so and told what to say to officials looking into the death.
Bowman said he "was told by his chain of command what version to give" investigators, Sergeant Irene Cintron of the Army's Criminal Investigation Command said during a teleconference from Iraq.
Involuntary manslaughter charges are still pending against a third enlisted soldier, 1st Sergeant Tracy Perkins, 33.
The three men from Fort Carson's 3rd Brigade Combat Team admitted forcing two Iraqi curfew breakers to jump into the river, though they said both men made it safely to shore.
Also on Tuesday, their commanding officer waived his right to a hearing to determine whether he will be court-martialled.
First Lieutenant Jack Saville was charged with involuntary manslaughter and attempting to cover up a death. He also faces an aggravated assault charge from a separate incident in December.
Saville's attorney did not return calls seeking comment.