Rapid eye movement (REM) sleep behavior disorder (RBD) is a preclinical feature of synucleinopathies, such as Parkinson's disease (PD).This study aimed to investigate the presence of potential early manifestations of parkinsonism, such as olfactory dysfunction and substantia nigra (SN) hyperechogenicity, in idiopathic RBD (iRBD) patients, PD patients and normal controls. We performed an olfactory function test using the cross-cultural smell identification test (CC-SIT) and midbrain transcranial sonography (TCS) in 15 patients with iRBD as confirmed by polysomnography, 30 patients with PD, and 30 normal controls. The CC-SIT scores of the iRBD patients and PD patients were significantly lower than those of the normal controls and similar between iRBD and PD (mean ± SD, 7.1 ± 2.2 and 7.6 ± 2.4 vs. 10.4 ± 1.2, respectively, p < 0.01). The sum of bilateral SN echosignals in the iRBD patients was greater than that of the normal controls but lower than that of the PD patients (0.29 ± 0.47, 0.11 ± 0.17 and 0.72 ± 0.41 cm(2), respectively, p < 0.01). In conclusion, we found that the concomitant abnormality of olfaction and increased SN echogenicity was more frequent in iRBD compared with normal control. Olfactory dysfunction and SN hyperechogenicity could be preclinical manifestations of parkinsonism in iRBD patients.