Hemoglobin (Hb) is the main component of red blood cells, and the majority of alpha-synuclein in the blood is present inside them. In Parkinson's disease and aging human brains, the levels of mitochondrial neuronal hemoglobin are significantly decreased in neurons where α-Syn has accumulated, suggesting a relationship between these two proteins. In the present study, we demonstrate that a complex comprising α-Syn and Hb exists in both peripheral RBCs and brain tissue in aging humans using Co-Immunoprecipitation and a newly established ELISA assay. The nHbα−Syn levels in the mitochondrial fraction of the human striatum reduced in an age-dependent manner which was negatively correlated with the nHbα−Syn complex levels in the cytoplasmic extraction. In contrast, no age-related alteration in the nHbα−Syn complex was found in the above subcellular fractions of the human cerebellum. Furthermore, the nHbα−Syn complex levels in RBCs increased with age, which agrees with the changes observed in the cytoplasm of the human striatum. Meanwhile, sequential window acquisition of all theoretical mass spectra-MS (SWATH-MS) was used to confirm the presence of the Hbα−Syn complex in RBCs and also found that nHbα−Syn levels increased notably in PD patients compared to healthy controls (HC). Receiver operating characteristic (ROC) curves indicated that Hbα−Syn in PD patients was distinct from HC, with areas under the curve (AUCs) of 0.875. The above results suggest that the alteration in nHbα−Syn levels in RBCs reflects those in select regions of the human brain, thereby providing a possibility of using Hbα−Syn as a biomarker to diagnose PD. © 2020 Elsevier B.V.