About 1,400 runners participated in the 10-kilometre race in Pluak Daeng district at 6am.
The two male runners -- Narasit Sansompak, 54, and Sakan Janthiya, 30 -- fainted after running for four kilometres.
The paramedic team tried to save their lives but it was too late. Rayong Hospital will perform an autopsy to find out the cause of death.