The (Lancaster) Intelligencer Journal/Lancaster New Era says 20-year-old Oscar Manuel Martinez entered the plea Wednesday in Lancaster County Court.
Defense attorney James Gratton said his client feared getting the death penalty in the April 2011 slaying of 33-year-old Diana Spencer and wanted to plead guilty and be sentenced immediately.
In court, Martinez said "All I can say is I did it and I confessed to it."
Police say the victim was shot through her home's front door after a group of men came looking for another man.
Prosecutors are also seeking the death penalty against two other men awaiting trial in the case.