The 16-year-old was followed from her home before being attacked without warning outside the Black Lion pub in Plaistow, east London.
The London Ambulance Service were called and the victim was taken to Newham General Hospital with bruising to the head and cuts to her face. She was treated and released that day.
Scotland Yard are appealing for information over the attack, which happened at 12.30pm on November 13, and have released shocking CCTV footage of the incident.
In the footage, a "black man of muscular build" is seen running up behind the girl before delivering a heavy right hook to the back of her head, knocking her unconscious and sending her face-first into the pavement.
Anyone with information should contact Newham Violent Crime Unit or Crimestoppers.
