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
A group of children from the South living with illness or disability have just returned from a special holiday in Orlando, Florida.
Simon's Blog - Ladybird Update
Wednesday night's weather for the east of the region