Medics rushed to treat the runner, who was in his 30s, after he completed the Southend Half Marathon in Shoeburyness yesterday, but he died soon after.
He had been among the 2,133 runners who had taken part in the race along Shoebury East Beach, Essex
The annual event, which has been held for more than 20 years, is organised by Havens Hospices Group.
A spokesman said: "The male runner collapsed at the finish. Our thoughts and prayers are with his family and friends at this incredibly sad time."