Iron metabolism is a research focus in α‑synucleinopathies. Excessive iron deposition may damage neurons and induce cognitive impairment. However, research on brain iron deposition in patients with idiopathic rapid eye movement sleep behavior disorder (iRBD) is lacking. Using quantitative susceptibility mapping (QSM), the present study showed that iRBD patients have greater brain iron deposition in the substantia nigra and dentate nucleus than healthy controls (HCs). Additionally, brain iron deposition in the striatum and cerebellum was associated with cognitive impairment, suggesting the potential of QSM as an auxiliary biomarker for neurodegeneration and the early evaluation of cognitive decline in iRBD patients.