The 84-year-old victim was shopping with his wife at the Sainsbury's supermarket on the afternoon of Wednesday, October 16.
Before going inside, police said he used the cashpoint just before 3pm. Once he got to the checkout at 3.30pm however, he realised that his wallet had been stolen.
Police said cash transactions for several hundred pounds, using both his and his wife's debit cards, were made at his bank in Worthing soon afterwards. Both cards have since been cancelled.
This man, described as white, in his thirties and wearing glasses, was seen near the victim at the cashpoint outside Sainsbury's, police said.
PC Martin Hunt said: "If you think you can put a name to this man, please contact us either online or by calling 101, quoting serial 540 of 17/10."