The incident occurred onboard a District line train during rush hour on November 11.
According to the British Transport Police, a woman boarded the service at West Ham shortly before 5.30pm.
Whilst on the train she noticed a man staring at her, resulting in her feeling uncomfortable.
As the train arrived at Upton Park just three minutes later, the man allegedly sexually assaulted her as he made his way off the train.
Detectives have shared an image of a man they believe may have information that could help with the investigation.
Anyone who recognises him is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference number 584 of 11 November. Alternatively you can call Crimestoppers anonymously on 0800 555 111.
