The relatively lenient sentence for Spc. Belmor Ramos was part of a deal that will see him testify against the others accused in the killings last year. The four Iraqi men were killed and dumped in a Baghdad canal in April 2007 - allegedly in retribution for casualties in Ramos' unit.
Ramos, 23, had faced a possible sentence of life in prison for conspiracy to commit murder.
Judge Lt. Col. Edward O'Brien said that, had it not been for the plea agreement, he would have sentenced Ramos to 40 years in prison. Ramos, of the 1st Battalion, 18th Infantry Regiment, will also have his rank reduced to private and be dishonorably discharged from the army.
His defense attorney, Capt. Patrick Bryan, who had asked for his client to be formally reprimanded and allowed to stay in the Army, had little to say about the sentence.
"It is what it is," he said, noting that the case would be automatically reviewed, a process akin to an appeal.
Three other soldiers in the unit - Sgt. John E. Hatley, Sgt. 1st Class Joseph P. Mayo, and Sgt. Michael P. Leahy Jr. - were charged Tuesday with premeditated murder, conspiracy to commit premeditated murder and obstruction of justice.
They face a hearing to determine if there is enough evidence for them to be sent before a court-martial, but no date has been set.
Ramos, of Clearfield, Utah, testified that he had agreed with the decision to kill the prisoners, and stood watch from the machine-gun turret of his Humvee while others carried out the killings. He said he heard the shots, but did not personally witness the deaths.
Ramos, along with Spc. Steven Ribordy, 25, waived his rights to the Article 32 hearing, the equivalent to a civilian preliminary hearing.
A date for Ribordy's court-martial has not been set.
In an earlier statement, the Army said the allegations related to "the deaths of several detainees who were captured as part of combat operations last year."
That statement, released in January, said that "preliminary findings indicate the deceased detainees were not persons detained in a detention facility," indicating the men were killed shortly after being captured.
In hearings in late August, soldiers who were on the patrol said the four Iraqis who were killed, probably Sunnis, were taken into custody following a shootout with insurgents, and taken to the unit's operating base near Baghdad.
Hatley and Leahy were also charged with one count each of premeditated murder and conspiracy to commit premeditated murder in a death near Baghdad in January 2007. Leahy was also charged with being an accessory after the fact, a statement this week from the military said.
The Army did not provide details on that death.