Police believe the same thief has struck at jewellers three times in the Brighton and Hove area in the past few weeks.
Sussex Police said that the search for the man had intensified while urging jewellery shop staff to be vigilant.
The suspect stole a £9,000 Breitling watch from Ernest Jones in Churchill Square, Brighton, at about 4pm on Friday 20 April.
He asked to try on the watch and as the shop assistant placed it on his wrist he grabbed it and ran out of the shopping centre towards Cannon Place.
Police believe that the same man walked into Pavilion Diamonds in Meeting House Lane in Brighton at about 4pm on Sunday 22 April.
He asked to view a £3,000 diamond ring. When the ring was in his hand he ran towards the door but was stopped by staff. He threw the ring back into the store before running off towards North Street.
The same man is believed to have gone into James Ross Jewellers in Station Road, Portslade, at about 4.15pm on Monday 23 April.
He asked to see a £3,750 Rolex watch. He stayed in the shop for about five minutes before leaning over the counter, taking the watch and running out of the store.
PC Richard Valder-Davis, from the Brighton Response Investigation Team, said: “This is a very brazen individual who has been walking into jewellery stores during the day intent on stealing very valuable items.
“We are focusing our resources on finding him and in the meantime I would advise jewellers to be vigilant and aware when showing items of jewellery to customers.
“We are also keeping an open mind that this same individual may be linked to the theft of a £3,000 diamond ring from a jewellers in North Road, Lancing, on Monday 30 April at around 4.40pm.
“Again, the suspect asked to look at the ring and then ran out of the store with it.”
The suspect is described as a slim white man, about 5ft 9in tall and with short blond or ginger hair.
When he stole the Breitling watch he was wearing dark jeans and a long-sleeved red jumper with the word Hollister written across the front and the sleeves pushed up.
He had a slight scar or mark on the left side of the upper lip and spoke with a southern accent.
Anyone who has information that could help police with their inquiries or who has been offered the stolen items for sale is asked to call Sussex Police on 101.
