Accurate antemortem diagnosis of parkinsonism is primarily based on clinical evaluation with limited biomarkers. We evaluated the diagnostic utility of quantitative rapid eye movement (REM) sleep without atonia analysis in the submentalis and anterior tibialis muscles in parkinsonian patients (53 synucleinopathy, 24 tauopathy). Receiver operating characteristic curves determined REM sleep without atonia cutoffs distinguishing synucleinopathies from tauopathies. Elevated submentalis muscle activity was highly sensitive (70–77%) and specific (95–100%) in distinguishing synucleinopathy from tauopathy. In contrast, anterior tibialis synucleinopathy discrimination was poor. Our results suggest that elevated submentalis REM sleep without atonia appears to be a potentially useful biomarker for presumed synucleinopathy etiologies in parkinsonism. ANN NEUROL 2019;86:969–974