In return for his plea in a courtroom at Joint Base Lewis-McChord court in Washington state, prosecutors agreed not to seek the death penalty for Bales in the March 2012 killings, NBC News reported.
A military jury will decide whether Bales, 39 -- who admitted to 16 counts of murder, six counts of attempted murder and seven counts of assault -- should ever be given a chance for parole. NBC said the married father of two pleaded not guilty to a charge concerning a stolen laptop.
Bales' attorneys have said he suffered from post traumatic stress syndrome and brain injury after four tours of combat and had been using drugs and alcohol the night of he made raids on family compounds in Kandahar province.
Prosecutors have said Bales was upset about an explosion near his outpost and the attacks were his response.