Introduction: REM sleep behavior disorder (RBD) is strongly associated with synucleinopathy and is caused by REM sleep without atonia (RSWA), the loss of normal muscle atonia during REM sleep. We aimed to determine whether RSWA severity was associated with cognitive functioning in RBD. Materials and methods: Both idiopathic (iRBD) and symptomatic RBD (sRBD) patients completed two cognitive batteries: CNS Vitals Signs (CNS-VS) and Useful Field of View (UFOV). All subjects underwent PSG and their muscle (SM: submentalis; AT: anterior tibialis) tone during REM sleep was visually and automatically scored. Group differences between sRBD and iRBD were then compared, and regression models fit to determine the relationship of RSWA and dependent cognitive measures. Results: Twenty iRBD and 10 sRBD participated. Demographics were similar between groups. Deficits on cognitive testing were observed on CNS-VS in processing speed (p = 0.014) and psychomotor speed (sRDB < iRBD, p = 0.019) and on Total UFOV and subtests 2 and 3 (sRBD > iRBD, all p < 0.002). sRBD patients had greater combined phasic and tonic RSWA in SM (p = 0.026) and longer mean phasic burst duration (p = 0.03). Regression analyses demonstrated that SM RSWA independently predicted overall CNS-VS Neurocognitive Index (NCI) (F = 4.5, p = 0.006), adjusting for age, gender, depressive symptoms (Zung score), and sleep disturbances (PSQI), and this relationship also remained significant in the iRBD group after excluding sRBD patients (F = 3.5, p = 0.03). Conclusion: SWA is predictive of lower overall cognitive performance in patients with RBD. Acknowledgements: The project described was supported by the National Institute on Aging (P50 AG016574), and through Grant Number 1 UL1 RR024150-01. The content is solely the responsibility of the authors.