According to British Transport Police, on Tuesday 19 October, a man boarded the 2.04pm service from Lewes to Brighton on Tuesday 19 October, and placed his bassoon in the overhead luggage hold.
As he alighted the train, Transport Police say he left the woodwind instrument behind and the train had departed when he returned to the platform.
The BTP say further enquiries show that the instrument was stolen by another passenger who then boarded a service towards Hastings and alighted at Southwick station.
Officers believe the man in the CCTV images may have information which could help their investigation.
If you recognise him, or have any information, please contact BTP by texting 61016 or calling 0800 40 50 40, quoting reference 2100079515.