The incident – involving a masked man – took place at David Smith Jewellery in the town’s high street at about 1:45pm yesterday (Wednesday 11, May).
Items including rings, necklaces and bracelets, with a total estimated value of £20,000, were taken during the raid.
The suspect is described as a man – about 6′ 3” and of skinny build – who was wearing a red and white mask, a black hoodie over his head and black jeans.
Sussex Police said the man also carried a hammer, which was used to smash two cabinets in the premises, and a black rucksack, which was used to carry the stolen items.
He was last seen heading down Station Street, where he turned right towards Southover Grange Gardens.
Detective Sergeant Jo Benton said: “The nature of this incident has had a distressing effect on the shop workers and we are working with them to try and identify the offender.
“Fortunately, it is an isolated incident and no one was harmed during the raid. It occurred during broad daylight, when there were plenty of people around.”
Anyone with information has been urged to contact Sussex Police on firstname.lastname@example.org or call 101, quoting serial 804 of 11/5.