The incident happened at 2.20pm last Sunday when the 12-year-old was followed up Langley Hill by her attacker.
Police say the well-built black man overtook her, stopped to block her path and stared at her, but when she told him to go away he grabbed her by the arm and swore at her.
He then lashed out with his foot, sending her crashing to the ground before kicking her again and then running off, up the hill towards Tilehurst..
The girl suffered bruised ribs but was treated by paramedics and did not need to go to hospital.
The attacker is black, in his early thirties, 6ft 3ins tall, of large build, with a Jamaican or West Indies accent. He had dark hair, short stubble and was wearing thick-rimmed black plastic glasses.
PC Shaun Bull said: "Our investigation is at an early stage and we are still trying to piece together what happened.
"We are keen to hear from anyone who witnessed the incident or has any other information which may assist us.
"In particular, we would like to hear from anyone who saw a man hanging around outside the shops at the bottom of Royal Avenue, Calcot."
Superintendent Robin Rickard, West Berkshire police commander, added: "I understand that there will naturally be concern in the local community about what occurred on Sunday, however incidents of this nature are very rare in this area and our local neighbourhood officers are carrying out additional patrols to provide extra reassurance.
"While we do not want to cause panic, we do think it would be prudent for people to be extra vigilant and take sensible personal safety precautions when out and about. Any suspicious behaviour should be reported to police immediately."
Call PC Bull with any information on 101 or Crimestoppers anonymously on 0800 555 111. Visit thamesvalley.police.uk/crprev/crprev-pers/crprev-pers-selfdef/crprev-pers-selfdef-techniques.htm for personal safety tips