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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Freezing conditions have caused 24 hours of cross-channel travel chaos - which is set to continue into tonight.
The icy conditions has put a strain on many services in the South, especially those that help the elderly.
School children in Hampshire are performing the story of Mary and Joseph using songs by Take That.