The alleged incident occurred at around 1:30pm on Tuesday lunchtime in the Market Cross Centre and involved a man offering jewellery for sale.
The man police would like to speak to in connection with the incident placed a gold chain around the neck of a man as he attempted to sell it to him. The victim declined to buy the jewellery and as the suspect removed the chain, he has stolen the victim’s own gold chain and left a cheap necklace in its place.
The suspect is described as being white and around 6ft tall. He was of a stocky build with close cropped, dark hair and a round face.
He was described as being smartly dressed wearing a light tan, bomber style, leather jacket. He spoke with an Eastern European accent.
PC Neil Morris of Selby Safer Neighbourhood Team, said: "We are advising people to be on their guard if they are approached by such a person, and would also like to hear from anyone who saw a man selling jewellery in the area or has been approached in a similar manner.
"This was an audacious theft and we are appealing for anyone who can assist our enquiries to please get in touch."
If you would prefer to remain anonymous, information can be passed to Crimestoppers on 0800 555 111.
Please quote the North Yorkshire Police reference number 12130192480 when passing on information about this crime.