The news of the guilty pleas comes amid US media reports that American officials were informed of abuses against Iraqi detainees by US personnel months before the notorious Abu Ghraib prison scandal broke.
Private first class Andrew Sting and Private first class Jeremiah Trefney, both 19, entered their pleas at a 14 May court martial in Iraq, according to a statement by the 1st Marine Expeditionary Force in Iraq released on Thursday.
The torture of the prisoner took place in early April, months after the Abu Ghraib prison abuse occurred.
Sting and Trefney were infantrymen with the 2nd Battalion, 2nd Marine Regiment, which is attached to the 1st Marine Division.
According to the military statement, Sting, Trefney and two other marines decided to shock a detainee at the al-Mahmudiya prison, a temporary holding facility, in order to discipline him for throwing trash outside his cell and speaking loudly.
The marines attached wires to a power converter, which was used to shock the detainee with 110 volts of electricity as he returned from a trip to the bathroom, the statement said.
Sting pleaded guilty to charges of assault, cruelty and maltreatment, dereliction of duty and conspiracy to assault. He was sentenced to a year in prison, a reduction of rank, forfeiture of pay and a bad conduct discharge.
The US army has been rocked by
images of prison abuse in Iraq*
Trefney pleaded guilty to cruelty and maltreatment, dereliction of duty, false official statement, violating a lawful order and conspiracy to commit assault.
He was sentenced to eight months in prison, reduction of rank, forfeiture of all pay, and will receive a bad conduct discharge.
The two other marines, who were not identified, are awaiting court action.
The pleas by Trefney and Sting came five days before the highly publicised court martial hearing for army reserve specialist Jeremy Sivits in the Abu Ghraib prison abuse scandal.
Abu Ghraib revelations
The first defendant to go on trial in the Abu Ghraib case, Sivits, 24, pleaded guilty to four counts of abuse and received the maximum sentence of one-year imprisonment, reduction in rank and a bad conduct discharge on 19 May.
But months before the Abu Ghraib scandal broke, a Chicago-based human rights group alerted US officials to the abuse of Iraqi detainees at that prison, the Chicago Tribune reported on Thursday.
A staff member of Christian Peacemakers Teams, which had people on the ground in Iraq, briefed middle-level and senior US Army officers on the abuses in the autumn of 2003, the daily said.
At least one official, Major Simmie Clincy, said the army was aware of the problems that the group had documented at Abu Ghraib and other detention centers in Basra, Ramadi and Tikrit.
Military authorities eventually launched an investigation in January 2004 but it wasn't until a US network ran photos of abused prisoners in April that the scandal came to public attention.
*Image courtesy of The Washington Post