REM sleep behavior disorder (RBD) has a tighter link with synucleinopathies than other neurodegenerative disorders. Parkinson’s Disease (PD) patients with RBD have a more severe motor and cognitive impairment; biomarkers for RBD are currently unavailable. Synaptic accumulation of α-Syn oligomers and their interaction with SNARE proteins is responsible for synaptic dysfunction in PD. We verified whether oligomeric α-Syn and SNARE components in neural-derived extracellular vesicles (NDEVs) in serum could be biomarkers for RBD. Forty-seven PD patients were enrolled, and the RBD Screening Questionnaire (RBDSQ) was compiled. A cut-off score > 6 to define probable RBD (p-RBD) and probable non-RBD (p non-RBD) was used. NDEVs were isolated from serum by immunocapture, and oligomeric α-Syn and SNARE complex components VAMP-2 and STX-1 were measured by ELISA. NDEVs’ STX-1A resulted in being decreased in p-RBD compared to p non-RBD PD patients. A positive correlation between NDEVs’ oligomeric α-Syn and RBDSQ total score was found (p = 0.032). Regression analysis confirmed a significant association between NDEVs’ oligomeric α-Syn concentration and RBD symptoms (p = 0.033) independent from age, disease duration, and motor impairment severity. Our findings suggest that synuclein-mediated neurodegeneration in PD-RBD is more diffuse. NDEVs’ oligomeric α-Syn and SNARE complex components’ serum concentrations could be regarded as reliable biomarkers for the RBD-specific PD endophenotype.