A woman was grabbed from behind and punched in the face by an attacker in Ystrad Mynach.
The attack happened at around 8pm last night, Sunday September 28, on Central Street, near the junction with Church Street.
Police said the victim did not require hospital treatment, but was left very shaken.
Officers are appealing for information to help identify the attacker, who is described as a white man, 6ft tall of slim build with short cropped dark hair.
He is thought to be in his late 20s and was wearing dark clothing at the time of the incident.
Anyone who might have witnessed the attack or has any information can call Gwent Police on 101 or Crimestoppers anonymously on 0800 555 111 quoting log 504 28/09/14.