His brother and co-defendant, Michael Bever, 17, did not enter and will be tried by a jury in June 2017, a court clerk said. Robert Bever could have faced the death penalty.
Michael Bever, who was 16 at the time of the killing, is charged as an adult but is ineligible for the death penalty due to his age.
The brothers are accused of bludgeoning and stabbing to death their mother, father, two brothers and a sister in Oklahoma in July 2015.
Parents David and April Bever, younger brothers Daniel, 12, and Christopher, seven and little sister Victoria, aged five, were murdered with multiple weapons including knives and a hatchet .
Sister Crystal, then aged 13, was critically injured but survived the attack at the family home in Broken Arrow, about 15 miles southeast of Tulsa, police said.
A two-year-old girl, identified as Autumn Bever, was found unharmed at the property.
A 911 call made by Daniel Bever led officers to a suburban home where they discovered the blood-splattered crime scene.
Robert and Michael Bever were tracked down by dogs in a wooded area near the house.
Police said the brothers told them they wanted to be known as famous criminals and launch a nationwide killing spree.