"The boy grabbed a gun on Saturday afternoon after his sister would not give up the controller", US police noted.
He allegedly shot her from behind, and the bullet entered her brain.
A local sheriff announced on Sunday that the teen had died of her injuries in a Memphis hospital.
It was unclear how the boy obtained the gun.
The children's mother was in another room, feeding other children lunch at the time of the incident.
Police are still investigating the circumstances of the shooting.
"This is all new ground for us, we've never dealt with a kid shooting a kid at age nine," Monroe County Sheriff Cecil Cantrell told local press.
Since 2013, there have been 291 reported school shootings in America, which averages to about one a week.
The data has been compiled by the Gun Violence Archive, a non-profit that tracks gun-related violence in the US.
According to the Gun Violence Archive, a non-profit that tracks gun-related violence in the US, since January, 69 children between the ages of 1-11 years and 333 teenagers (12-17 years) have been killed in various incidents.
The semi-automatic weapon, which was introduced by Colt in the 1960s, has virtually become the favorite weapon for mass murderers across the US.
Last October, a gunman armed with 14 AR-15s and a cache of other deadly weapons opened fire on a music festival from his nearby hotel room leaving 58 people dead and 851 injured.
In September 2016, a gunman used stormed a nightclub in Orlando, Florida, Killing 49 people and wounding 58 others using his AR-15.
