Neurosin is a protease that in vitro degrades alpha-synuclein, the main constituent of Lewy bodies found in brains of patients with synucleinopathy including Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). Several studies have reported reduced cerebrospinal fluid (CSF) levels of alpha-synuclein in synucleinopathy patients and recent data also proposes a significant role of alpha-synuclein in the pathophysiology of Alzheimer’s disease (AD). To investigate potential links between neurosin and its substrate alpha-synuclein in vivo we used a commercially available sandwich ELISA and an in-house developed direct ELISA to quantify CSF levels of alpha-synuclein and neurosin in patients diagnosed with DLB, PD and PD dementia (PDD) versus AD patients and non-demented controls. We found that patients with synucleinopathy displayed lower CSF levels of neurosin and alpha-synuclein compared to controls and AD patients. In contrast, AD patients demonstrated significantly increased CSF alpha-synuclein but similar neurosin levels compared to non-demented controls. Further, CSF neurosin and alpha-synuclein concentrations were positively associated in controls, PD and PDD patients and both proteins were highly correlated to CSF levels of phosphorylated tau in all investigated groups. We observed no effect of gender or presence of the apolipoprotein Eepsilon4 allele on neither neurosin or alpha-synuclein CSF levels. In concordance with the current literature our study demonstrates decreased CSF levels of alpha-synuclein in synucleinopathy patients versus AD patients and controls. Importantly, decreased alpha-synuclein levels in patients with synucleinopathy appear linked to low levels of the alpha-synuclein cleaving enzyme neurosin. In contrast, elevated levels of alpha-synuclein in AD patients were not related to any altered CSF neurosin levels. Thus, altered CSF levels of alpha-synuclein and neurosin in patients with synucleinopathy versus AD may not only mirror disease-specific neuropathological mechanisms but may also serve as fit candidates for future biomarker studies aiming at identifying specific markers of synucleinopathy.