The teen entered the guilty plea in Contra Costa County Superior Court earlier this month in connection with the brutal beating that led to the death of 83-year-old Patrick Chang, according to Senior Deputy District Attorney Dan Cabral.
The defendant, whose name is not being released because he is a juvenile, is in custody and awaiting sentencing.
Police said the teen beat Chang during a robbery in the parking lot an Orchard Hardware Supply store at 4873 Lone Tree Way on May 18.
Five days later, officers arrested the then suspect, who was then 15 years old.
After Chang died from his injuries in June, the teen was charged with murder.
In a statement on the Antioch Police Department's Facebook page on Tuesday, police Chief Allan Cantando said community members' tips based on a surveillance photo of the suspect led to the suspect's capture and prosecution.
"This is yet another example demonstrating the power of the close relationship and the constant communication shared between this Police Department and the community it serves," the chief wrote.