The 15-year-old boy was hit by a grey sedan at Chapel Road, Bankstown, just before 4pm, police said.
The boy suffered serious pelvic and leg injuries.
He has been taken to The Children's Hospital at Westmead in a serious condition.
The grey sedan fled the scene and now police are searching for the driver.
The incident has subsequently caused traffic chaos in the area with southbound travellers being diverted into Eldridge Road.
Commuters are urged to leave extra time if travelling in the area.