The attack involved a woman being punched in the face and knocked to the floor.
The attack took place on a late-night train from Brighton to Worthing and now the man is being hunted by the British Transport Police (BTP).
His image – caught on CCTV – is numbered nine out of ten released by the BTP as they try to solve a range of crimes across the south east.
Officers want to identify him and speak to him about the attack which took place on Sunday 1 May on the 11.30pm Brighton to Worthing train.
The BTP said that the public should not approach the man pictured.
Anyone who knows his identity is urged to call the BTP on 0800 405040 or the independent charity Crimestoppers anonymously on 0800 555111.
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