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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Amanda Houston has the latest forecast for the west of the ITV Meridian region
Amanda Houston has the latest forecast for the east of the ITV Meridian region
The brother of a Kent sailor killed during the Clipper Round the World yacht race says being aboard his boat will be emotional.