The single-stone, platinum ring valued at £29,500 was taken from Timothy Roe Fine Jewellery on South Street on Thursday afternoon.
Police are investigating and want to speak to a man pictured by the store’s CCTV cameras in connection with the theft.
The stolen ring was the most valuable item on display and staff said the thief simply ran out of the door with it after asking how much it was worth.
Director Dan Roe said: “He came in between 2.30pm and 2.40pm and asked to see a ring in the window.
“He had another diamond ring with him and asked if he could part exchange it, but Rowena (sales assistant and designer) told him we don’t generally do that.
“He then asked what the best price we could do the ring he had seen in the window ring for.
“She left him with another member of staff to come up stairs to speak to us, and while she was gone he just turned around and ran out the door.
“It’s a real shock. We’ve been very lucky that in 20 years of being here we have never had anything like this happen, certainly not on this scale.
“We’ve had a couple of bracelets taken but nothing like this, it was a really large diamond.”
The CCTV images were put on the Timothy Roe Fine Jewellery Facebook page soon after and the appeal has been shared more than 600 times.
“There’s been a really good response on Facebook and we’re just hoping someone recognises him,” Mr Roe said.
“It was the biggest ring in the window, it’s a real hit, especially at this time of year.”
Police believe this incident could be linked to another theft from a jewellery store in Petersfield, Hampshire earlier that afternoon.
The suspect is described as white, about 6ft, of medium build and aged between 45 and 55. He was wearing glasses, a tartan trilby, black coat and striped scarf.
PC Dave Phillips said: “This theft happened in front of shop staff in a busy part of Chichester during daylight hours.
“We would like to speak with the man shown in the image and anyone who may have seen what happened.”
Sales assistant and designer Rowena added: “He was quite cocky, as I was going up the stairs he shouted ‘I’ll give you £28,000 for it’.
“He was confident but he was also shaking loads.
“I’ve been working here for seven years and nothing like this has ever happened, it really sucks.
“I just feel so bad for Tim and Dan (Roe) who are lovely, we’re an independent jewellers not a massive chain store.”
Witnesses can email [email protected] or call 101 quoting reference 0795 of 26/11.
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