Urinary urgency and frequency are common in α-synucleinopathies such as Parkinson disease, Lewy body dementia, and multiple system atrophy. These symptoms cannot be managed with dopamine therapy, and their underlying pathophysiology is unclear. We show that in individuals with Parkinson disease, Lewy body dementia, or multiple system atrophy, α-synuclein pathology accumulates in the lateral collateral pathway, a region of the sacral spinal dorsal horn important for the relay of pelvic visceral afferents. Deposition of α-synuclein in this region may contribute to impaired micturition and/or constipation in Parkinson disease and other α-synucleinopathies.