Police have launched an appeal for information following the alleged incident, which is reported to have happened on a train on Saturday, April 30.
The alleged victim said boarded a train at 6pm at Cardiff Central, before a man stood next to her and sexually assaulted her.
The man is described as being white, approximately 50 years old and of medium build. He had short, grey facial hair and was wearing dark blue jeans, a white polo shirt, blue hooded jacket, white trainers, a black cap and glasses.
British Transport Police said the train was busy at the time as fans were travelling following the Cardiff City v Birmingham City football match that happened in the city that afternoon.
Anyone who may have any information should contact the British Transport Police by calling 0800 40 50 40 or texting 61016. Anyone contacting the force for this investigation should use the reference 584 of 30/04/22.
Crimestoppers can also be contacted anonymously on 0800 555 111.