The teen was arrested and charged as a juvenile with aggravated assault, simple assault, recklessly endangering another person and weapons offenses.
The shooting happened around 7:30 p.m. on April 7 in the 200 block of S. 58th Street.
According to the district attorney's office, a total of four children ages 12 to 13 were inside a home without any adults around.
Investigators say one of the children found an unsecured gun, and some or all of the children took turns touching it.
At some point the gun went off, striking the 12-year-old girl in the abdomen.
At least one of the kids applied pressure to the wound and called for help.
Officials say the girl is in stable condition at the hospital.
The gun has been recovered, the D.A.'s office said, adding that the investigation into this shooting continues.