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.
More top news
Wednesday morning's weather for the east of the region
Wednesday morning's weather for the west of the region
The first of 1,400 new commuter train carriages for the south has just arrived from Germany.