A spokeswoman for the Flora London marathon organising committee said: "It is with regret that we can confirm that a 22-year-old man died peacefully this morning having collapsed after successfully completing the marathon.
"We offer our deepest sympathy and condolences to his friends and relatives."
She added that organisers "fully respect" the wishes of his family not to release any more details about the fatality at this time.
Yesterday's race was run in difficult conditions with temperatures reaching 20.5 C in some parts of the course.
Organisers say it was the biggest ever race in its 27-year history, with at least 36,396 people starting the marathon in Blackheath, east London, and 35,674 having finished by 18:30 BST last night.
Kenya's Martin Lel took the men's title in a time of two hours, seven minutes and 41 seconds, while China's Chunxiu Zhou took the women's title in 2:20:38.
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.