The incident happened at around 6.30pm on Sunday evening in the Spandau district, Tagesspiegel reports.
The child was cycling with the aid of training wheels on the pavement when he smacked head-on into the old lady.
The 83-year-old fell into a parked motorbike, fracturing a bone in her upper thigh and sustaining serious head wounds.
She was taken to hospital in an ambulance for further treatment.
The small child, who was in the company of an adult woman, did not wait at the site of the accident for the police but instead cycled away from the scene.
Police are now looking for the small boy and his companion.