Police were called to Loxham Road at around 3.30pm yesterday (October 15).
Officers arrived with paramedics who treated the boy at the scene.
He was taken to hospital with injuries thought to be life-threatening.
There have been no arrests and police are asking anyone with information to get in touch.
Chingford Road was closed following the crash but has since reopened.
A Waltham Forest council spokesperson said after the incident: “Due to a traffic incident, Chingford Road is closed to car traffic between Walthamstow Avenue and Loxham Road.
“If you can, please avoid the area and use an alternate driving route.”
A London Ambulance Service spokesperson said: “We were called today (Weds) at 3.30pm to reports of a traffic collision in Loxham Road, E4.
“We sent an ambulance crew and treated a child at the scene before taking him to a London major trauma centre.”
A Met Police spokesperson said: “Around 15:30hrs on Wednesday, 15 October, officers attended Loxham Road, Chingford, alongside the London Ambulance Service, after a car collided with a 12-year-old boy.
“The child was treated for injuries believed to be life-threatening. They were taken to hospital, where they remain.
“No arrests have been made. Anybody who witnessed the incident is urged to call 101, quoting reference 4935/15OCT.”
