We quantified substantia nigra (SN) damage in idiopathic REM sleep behavior disorder (iRBD) patients using multimodal MRI biomarkers and determined biomarker efficacy. Nineteen patients with iRBD and 18 healthy volunteers underwent 3-Tesla MRI, including diffusion tensor imaging, neuromelanin (NM)-sensitive imaging and T2* mapping. The volume and normalized signal intensity in NM-sensitive images, R2* and diffusion tensor measures were quantified in the SN. Patients with iRBD showed reduced NM-sensitive volume and signal intensity and reduced fractional anisotropy versus controls in the SN. Combination of the three biomarkers had excellent diagnostic accuracy. These measures may represent valuable biomarkers for prodromal Parkinson’s disease.