Former Crystal Palace chairman Simon Jordan was robbed at gunpoint on Wednesday, having a £100,000 watch taken by a masked man on a motorbike wielding a gun.
The 50-year-old said the suspect pulled up alongside him at traffic lights in Croydon at about 13:00 GMT, and said the watch, bought to celebrate Palace's promotion, held "sentimental value".
Jordan said, as quoted by BBC News: "I was just sat in my car when I heard someone scream through my window and demanded I gave him my watch and pointing a gun in my face.
"I'm pleased nobody was hurt but the watch held sentimental value - I bought it to celebrate Crystal Palace's promotion to the Premier League."
Jordan said he was taking his father to a doctors appointment when he was confronted.
Police have urged anyone who is offered "rare, white gold, blue-faced Franck Muller watch" to contact them.
DS Tim Walter added: "This was a terrifying ordeal and robbery of a man sat in his car waiting at a traffic light.
"My team are doing everything they can to identify the culprit and bring him to justice. This involves the recovery of local CCTV footage and ongoing forensic work.
"The item stolen is a rare white gold, blue faced Franck Muller watch. If you are offered this item for sale or have on information on its whereabouts I would urge you to get in touch."
Jordan was Palace's chairman between 2000 and 2010, becoming the youngest chairman of a Football League club at the age of 32.