The BTP said the victim boarded the train at Three Bridges station with his suitcase which contained a quantity of cash at around 7am on Monday, February 3.
At around 9:30am, BTP said a woman picked up the suitcase and left the train at St Pancras railway station.
Officers believe the woman in the image in this article may have information which could help the investigation.
If you recognise her, or have any information, please contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 428 of 8 February.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.
1. Suitcase which contained ‘quantity of cash’ stolen from commuter who boarded at Three Bridges station
Officers investigating the theft of a suitcase on board a Thameslink train are releasing this CCTV image in connection Photo: British Transport Police