The accident happened at around 4pm on Tuesday in the Castle Heights estate in Carrick-on-Suir.
It is understood the girl was playing near her home when she was hit by a bus.
She was pronounced dead a short time later. Her body has been taken to South Tipperary General Hospital where a post mortem will take place.
The scene has been preserved for a forensic examination.
Gardaí at Carrick-on-Suir are appealing for witnesses or anyone with mobile phone or dashcam footage to contact them.