The offender is about 6ft, and was wearing black clothing and had his face covered. He turned left after leaving the shop.
Anyone with any information should contact Det Sgt Homes via the 24-hour Police Enquiry Centre on 101. If you don't want to speak directly to the police, you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or online at crimestoppers-uk.org.
More top news
This year's Brighton Digital Festival is underway with events taking place across the city.
Saturday evening's weather update for the east
Saturday evening's weather update for the west