Fatigue is one of the most common and disabling symptoms of Parkinson's Disease (PD). The occurrence and clinical features of fatigue in patients with prodromal PD remain largely elusive. This study aimed to investigate the prevalence and clinical characteristics of fatigue in patients with idiopathic/isolated REM sleep behavior disorders (iRBD).A total of 97 polysomnography-confirmed iRBD patients were enrolled in this study. A comprehensive neurological assessment (including motor and non-motor assessment) was performed. Fatigue was assessed using the Fatigue Severity Scale (FSS). Motor and non-motor characteristics were compared between iRBD patients with and without fatigue. Logistic regression was used to identify the factors associated with fatigue.The prevalence of fatigue was 35.05%. Compared to the non-fatigue patients, patients with fatigue had higher non-motor symptom scale (NMSS) score (p = 0.009), higher Hamilton Depression Rating Scale (HAMD) score (p = 0.002), and a higher prevalence of orthostatic hypotension (p = 0.021). Multivariate regression analysis showed that depression (OR 4.17, 95% CI 1.13-15.49, p = 0.033) and orthostatic hypotension (OR 2.80, 95% CI 1.09-7.18, p = 0.032) were significantly associated with fatigue in iRBD patients. Additionally, both NMSS (rs = 0.310, p = 0.002) and HAMD (rs = 0.385, p < 0.001) scores were mildly correlated with fatigue severity.Our study showed that fatigue is common in patients with iRBD. In addition, depression and orthostatic hypotension were independently associated with fatigue in iRBD patients.