A spokesman for the Royal Sussex County Hospital said the man died at 2.30pm.
England cricketer Matt Prior had officially started today's race, the fourth Brighton marathon, in Preston Park at 9am.
Organisers said 9,315 competitors were taking part.
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Friday afternoon's forecast for the east of the region
Friday afternoon's forecast for the west of the region
South Central Ambulance service is about to embark on a controversial trial of the use of adrenaline versus placebo during cardiac arrest.