The man, aged 37, was taking part in the four-mile (6.5km) Loughrigg Fell Race near Ambleside on Wednesday night when he had a cardiac arrest.
He was given CPR by other athletes, but the Great North Air Ambulance Service said "despite everyone's best efforts", he was pronounced dead at the scene.
Event organiser Ben Abdelnoor from Ambleside Athletics Club said his thoughts were with the man's family.
The competitor was about 20 minutes into the annual race when he collapsed.
A man has died after collapsing during a fell race in the Lake District.