They were both in a Dodge SUV stopped at Hasting Circle when the boy found a gun in the backseat floorboard and fired it, said Easley police Maj. Tim Tollison.
The bullet went through the passenger seat, striking Alexandra Anita Santos, 22, of 400 South D St., according to Pickens County Coroner Kandy Kelley.
Santos died at 12:53 p.m. of a single gunshot wound to the chest, Kelley said.
Two other children ages three and five were also in the vehicle but weren't injured, Tollison said.
None of the children were related to the victim, he said.
Police are still investigating how the weapon, a semi-automatic handgun, wound up in the vehicle, Tollison said.