The 16-year-old was found suffering from a gunshot wound to the chest in the backyard of a residence just before midnight in the 12000 block of Burr Street, according to the Lake County Sheriff's Office.
An officer began administering CPR but the victim showed no signs of life. He was pronounced dead at the scene.
Police said the initial investigation determined the victim and an 18-year-old were in the backyard with friends when the 18-year-old pulled out a gun as a prank and it accidentally discharged, striking the boy in the chest.
The 18-year-old was taken to Saint Anthony's Hospital for an evaluation. He was later taken into custody on probably cause charges, police said.