Austin Sigg pleaded guilty on Tuesday to kidnapping and murdering Jessica Ridgeway.
Sigg, 18, was accused of committing the crimes one year ago.
He entered the plea agreement as part of a deal hours before opening statements were scheduled to begin in the trial in Colorado, reported ABC.
The plea was against his attorneys’ advice. It included all counts.
Sigg now faces a life sentence in prison on the murder charge alone.
Ridgeway was kidnapped as she walked to school on Oct. 5, 2012. She was found dead five days later.
Detectives said previously that Sigg confessed to killing the fifth-grader, but denied sexually assaulting her.
Sigg can’t get the death penalty because he was 17 at the time of the slaying.
Sigg was arrested after his mother called police. She said he turned himself in.
Sentencing is scheduled for November 18 and 19.
The Associated Press contributed to this report.