Isolated/idiopathic rapid-eye movement (REM) sleep behavior disorder (iRBD) is, in most cases, an early form of α -synuclein-related neurodegenerative diseases, including Parkinson's disease and dementia with Lewy bodies. Clinical reports suggest that iRBD is more common in individuals with Post-Traumatic Stress Disorder (PTSD) compared to those without PTSD. We conducted polygenic risk score (PRS), genetic correlation, and Mendelian randomization analyses to explore potential genetic and/or causal associations between PTSD and iRBD. Dopamine transporter imaging binding status was also examined in iRBD patients with (N = 6) and without PTSD (N = 32). While not supporting a causal relationship, genetic analyses revealed a significant association between PTSD and iRBD, consistent with the exploratory imaging substudy. These findings suggest that individuals genetically at risk for PTSD may also be at higher risk for iRBD. Further investigation of iRBD in individuals with PTSD may help inform potential neurodegenerative risk.