Scientists found cycling led to greater connectivity between brain regions linked to the disease.
But vigorous "forced" pedalling was key to the changes, which were associated with improvements in co-ordination and balance.
The US research lends support to anecdotal evidence of cycling alleviating Parkinson's symptoms. The debilitating disease, which affects motor regions of the brain, causes shaking, rigidity and slow movement.
Neuroscientist Dr Jay Alberts conducted the study after riding a tandem bicycle across Iowa with a patient to raise awareness of the disease.
He noticed his companion's condition improved after the ride.
Dr Alberts, from Cleveland Clinic Lerner Research Institute, said: "The finding was serendipitous.
"I was pedalling faster than her, which forced her to pedal faster. She had improvements in her upper extremity function, so we started to look at the possible mechanism behind this improved function."
The scientists found faster cycling boosted nerve connections between the primary motor cortex and thalamus, which is vital to co-ordinated movement, and impaired in Parkinson's patients.