Dekraai allegedly entered the Salon Meritage on October 12, 2011, and fatally shot his wife, Michelle Fournier, a hairdresser there.
Dekraai allegedly shot seven others inside the business, including the salon's owner, and a ninth victim in the parking lot. One person shot inside the salon survived.
Minutes after the shooting began, Dekraai was stopped by police and arrested. He told the officer "'I know what I did,'" according to police.
Dekraai was wearing a bulletproof vest and was carrying three guns when he was arrested. He told investigators he was in a custody dispute with his estranged wife, Fournier, over his 8-year-old son.
Dekraai pleaded not guilty in January 2012. His trial is scheduled to begin on June 9.
Scott Sanders, an assistant public defender and Dekraai's defense attorney, wants the death penalty thrown out. Sanders wants the Orange County District Attorney's Office to recuse itself because of recordings made by a fellow inmate that prosecutors characterized as Dekraai bragging about the killings. The district attorney's office agreed last week to drop efforts to use the recordings in court proceedings.
Dekraai, a former tugboat operator who lived in nearby Huntington Beach, had previously offered to plead guilty in exchange for multiple life sentences but prosecutors refused to drop the death penalty from consideration.
Sanders told an Orange County court during a hearing Monday that his client would enter the plea.
Sanders said in a statement: "Mr. Dekraai knows that his actions have caused terrible pain for the families and loved ones of the victims in this case. He also recognizes that the litigation surrounding the motion to dismiss the death penalty has added a sense of uncertainty about the proceedings . Mr. Dekraai wants the victims to have the certainty of knowing that he will never be released from prison. For these reason, Mr. Dekraai will plead guilty to each of the charges, the special circumstance allegations and the enhancements."
The shooting is the worst mass murder in Orange County history.
The Associated Press contributed to this report.