Staff Sgt. Frank Wuterich of Meriden, Conn., pleaded guilty to dereliction of duty, interrupting his trial at Camp Pendleton.
When he is sentenced on Tuesday, he is expected to receive a maximum of three months confinement, two-thirds forfeiture of pay and a rank demotion. Wuterich was originally charged with nine counts of manslaughter, but those charges are expected to be dropped.
Wuterich led the Marine squad in 2005 that killed 24 Iraqis in Haditha after a roadside bomb exploded near a Marine convoy, killing one Marine and wounding two others.
Prosecutors accused Wuterich of losing control after seeing the body of his friend blown apart by the bomb and leading his men on a rampage in which they stormed two nearby homes. Among the dead were women, children and elderly, including a man in a wheelchair.
Wuterich has said he regretted the loss of civilian lives but believed he was operating within military combat rules.
The plea deal ends the largest and longest-running criminal case against U.S. troops to emerge from the Iraq War. Six other squad members have had charges dropped or dismissed. One man was acquitted.
The Associated Press contributed to this report.