The assault happened in Merthyr Dyfan sometime between 5.30am and 8am on Sunday, July 12.
A woman was walking home from a nearby shop when she was assaulted by an unknown man.
Police said a thorough investigation, including CCTV examination and house-to-house enquiries, has been carried out since the incident was reported.
Detectives are now appealing for information from the public.
The suspect is described as white, aged in his 20s, between 5ft 4in and 5ft 6in tall, and of stocky build. He has light brown hair and was clean shaven.
He was holding a can of lager and wearing grey tracksuit bottoms.
No further similar incidents have been reported in the area.
Anyone who thinks they recognise the suspect, or has information or footage, is asked to contact South Wales Police, quoting reference 2600222188.
Information can be provided online at https://bit.ly/SWPProvideInfo, by calling 101, or anonymously through Crimestoppers on 0800 555 111. Always call 999 in an emergency.
South Wales Police said it takes all reports of sexual violence extremely seriously and that it is never too late to report a crime.
The force said victims can come forward no matter how long ago the offence happened. It said support is available whether people are ready to report an offence to police or would prefer to talk to someone else about it directly.
