Abstract Objective Rapid eye movement (REM) sleep behavior disorder (RBD) may be a risk factor for dementia development in patients with Parkinson’s disease (PD); however, the role of subclinical RBD remains unknown. Patients with PD and clinical RBD, subclinical RBD, or with normal REM sleep were examined in a cross sectional study and a longitudinal follow-up. Methods Interviews regarding RBD symptoms and polysomnographies were performed on 82 PD patients divided into RBD subcategories based on the presence/absence of REM sleep without atonia (RWA) and/or RBD symptoms. Descriptive variables were compared and patients were followed-up longitudinally for 21.4 ± 10.8 months. Results The existence of RBD, but not subclinical RBD, was associated with orthostatic hypotension and levodopa dose equivalents (LDEs) in patients with PD. Kaplan–Myer curves indicated that the occurrence of dementia in the PD group with clinical RBD was significantly faster than in the PD group with normal REM sleep ( p = 0.013). A Cox hazard regression analysis revealed that development to PD with dementia was only significantly associated with the presence of clinical RBD (hazard ratio: 14.1, p = 0.017). Conclusion Clinical RBD symptoms, but not subclinical RBD, were associated with the development of dementia in PD.