The distinctive pre-owned watch, valued at £720, had been in a window display at Tim Hogarth Jewellery and Antiques in Leeds' Central Arcade, off Briggate.
Furious business owner Tim Hogarth, who makes regular appearances on the TV show Dickinson's Real Deal, said the shirt-and-tie-wearing thief had come in shortly before 1pm and had asked to take a closer look at the watch before fleeing with it.
He said: "I had just handed him the watch and he just ran with it towards the Corn Exchange. There were another two guys stood outside and they ran in the other direction.
"He was about 5ft 8ins, in his early 20s and quite well-dressed. He had a bomber jacket, white shirt and tie and sandy blond hair.
"To be honest, he just looked like an office worker on his lunch hour."
Mr Hogarth said the stainless-steel watch had a white leather strap, two dials and was "heavily encrusted with diamonds".
He said: "It is very, very distinctive. It would be very difficult to sell."
Mr Hogarth said it was the second 'snatch' theft the premises had suffered, after another thief made off with a gold chain and pendant a few months ago.
He said: "We are just a small business and it affects us.
"We don't want to have a policy where we have to lock the door to show people stuff, but obviously it looks like it is getting towards that."
A police spokesman said anyone who had seen the theft, or had any information, could contact them on 101 or call Crimestoppers, anonymously, on 0800 555 111.