Fernhurst Junior School pupil Tia Gash was left in tears after the attack in Goldsmith Avenue, Southsea, at about 3.30pm on Wednesday, March 27.
She had been stood with her bike on the path close to Orchard Park when a man she did not know approached from behind and slapped her across the face.
The impact was so hard her earring came out and she sped to her Somers Town home to alert her parents.
Her older sister launched a Facebook appeal for witnesses which was shared more than 3,000 times, but no one has been taken into custody.
Speaking to The News Tia’s family told of their anger that ‘a grown adult could do that to their daughter’.
In a police appeal, officers described her attacker to be Asian, aged between 30 to 40 and between 5ft 8ins and 5ft 10ins tall.
Tia herself said the man was wearing a blue top with blue jeans and he was accompanied by a woman, who was wearing a pink top.
PC Matthew Campbell, said: ‘This was an unprovoked attack where a young girl has been assaulted.
‘We need to speak to anyone who witnessed this incident or anyone who thinks they may know who was responsible.’
Anyone with information should call police on 101, quoting the crime reference number 44190106410.