An "opportunistic burglar" took advantage of the warm weather as he raided a property as the owner was in the garden.
A large amount of jewellery was stolen in the burglary at a home in River Lane, Alfriston, after the burglar entered through the open back door at 12.25pm on Saturday.
A wallet, two wooden jewellery boxes and an oval silver and blue velvet trinket box were taken.
The jewellery boxes and trinket box contained a large variety of silver, gold diamond and gem jewellery as well as some costume jewellery which was also stolen.
There is believed to be one suspect who is described as a white man with a slight Asian appearance, aged 24, 5' 10" and of muscular build with short light brown hair.
He was clean shaven and wearing a close fitting dark jacket with blue jeans. It is believed he hid the stolen items under his jacket which would have made it look bulky.
Detective Constable Richie Davidson said: "An opportunist burglar has taken advantage of good weather and targeted a property whose occupant was outside in the back garden when he entered through an open back door.
"I would urge anyone who saw the man running away from the scene over PlonkBridge (also known as White Bridge) towards Lullington Road to contact police."
Sussex Police has launched Operation Magpie to tackle burglary across Sussex.
Any witness or anyone with information is asked to call 101 or email firstname.lastname@example.org quoting serial 728 of 05/07. Alternatively, call the independent charity Crimestoppers anonymously on 0800 555 111.