The collision happened, on the B5289 between Keswick and Borrowdale, at about 5.30pm on Saturday.
The 17-year-old boy injured was from out of the area.
He was flown to the Royal Victoria Infirmary, Newcastle, by the Great North Air Ambulance.
The rider of the motorbike was a 24-year-old man from west Cumbria.
A spokesman for Cumbria police said:" The pedestrian is in a stable condition."
The road was closed for several while collision investigators examined the scene.
Witnesses should call police on 101.
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