Sussex Police confirmed that a 72-year-old man from Worthing died outside WHSmith in South Street, Worthing, on Saturday afternoon.
A spokesman for the South East Coast Ambulance Service said they were called to the scene at 1.10pm, responding to reports that a person had collapsed.
Before they arrived, members of the public had attempted to help the man, including one person who found a public access defibrillator.
The air ambulance was also called, landing on Worthing Beach near the pier, and the crew helped paramedics with resuscitation efforts.
Sussex Police put a cordon around the scene, encompassing much of the WHSmith store, and police cars and ambulance vehicles were parked to block the thronging crowds nearby.
The ambulance spokesman said: "Thank you to everyone that helped, but sadly, despite everyone's best efforts, the person did die at the scene."
The man was taken away from the scene by road, and paramedics left the scene at 2.40pm.