<p>Neuromelanin-sensitive magnetic resonance imaging (NM-MRI) enables non-invasive assessment of the substantia nigra pars compacta (SNpc). However, its utility in identifying dopaminergic dysfunction in idiopathic rapid eye movement sleep behavior disorder (iRBD) remains unclear. This study evaluated NM-MRI alterations in the SNpc, stratified by dopamine transporter (DaT) imaging status, to assess its performance in detecting early dopaminergic alterations. This combined retrospective and prospective study included 66 patients with iRBD (45 males, 67.0 ± 6.3 years) and 30 healthy controls (HCs) (11 males, 63.6 ± 6.5 years). Patients with iRBD were divided based on ¹⁸F-FP-CIT PET findings into those with abnormal (iRBD-CIT+) and normal (iRBD-CIT−) DaT imaging. NM metrics, including volume, signal-to-noise ratio (SNR), and contrast-to-noise ratio, were quantified using a template-based semi-automated method in the whole SNpc and its subregions (sensorimotor, associative, and limbic). Among the 66 patients with iRBD, 37 (56.1%) were classified as iRBD-CIT+. They exhibited significantly reduced NM metrics across the whole SNpc and all subregions compared to HCs. Notably, NM volume and SNR in the associative and limbic area were significantly lower in the iRBD-CIT+ group than in the iRBD-CIT− group, whereas other metrics did not differ. ROC analysis revealed fair performance for NM volume and SNR (AUC 0.73 and 0.75, respectively) in discriminating iRBD-CIT+ from iRBD-CIT−. Multivariable regression analyses confirmed these metrics as independent discriminators of DaT abnormalities. NM-MRI metrics, particularly limbic NM volume and SNR, serve as potential biomarkers for dopaminergic alterations in iRBD. These findings support NM-MRI as a strategic, radiation-free triage tool to identify high-risk patients requiring DaT imaging in iRBD.</p>