He is thought to have had a cardiac arrest around the 16-mile mark of the course.
A Sussex Police spokesman said: "A 23-year-old man has sadly died after suffering a medical episode whilst competing in the Brighton marathon this morning.
"Emergency services were called to Grand Avenue in Hove at around 11.45am where it is believed the London man suffered a cardiac arrest.
He was treated at the scene by medical staff, including doctors and paramedics, along with an off-duty police officer, before being taken to hospital."
The man's family has been informed of his death.
A spokesman for organisers of the marathon said: "It is with regret that we can confirm that a competitor collapsed and later died competing in the Brighton Marathon this morning.