Background: Red blood cells (RBCs) contain the majority of α-synuclein (α-syn) in blood, representing an interesting model for studying the peripheral pathological alterations proved in neurodegeneration. Objective: The current study aimed to investigate the diagnostic value of total α-syn, amyloid-β (Aβ1–42), tau, and their heteroaggregates in RBCs of Lewy body dementia (LBD) and Alzheimer’s disease (AD) patients compared to healthy controls (HC). Methods: By the use of enzyme-linked immunosorbent assays, RBCs concentrations of total α-syn, Aβ1–42, tau, and their heteroaggregates (α-syn/Aβ1–42 and α-syn/tau) were measured in 27 individuals with LBD (Parkinson’s disease dementia, n = 17; dementia with Lewy bodies, n = 10), 51 individuals with AD (AD dementia, n = 37; prodromal AD, n = 14), and HC (n = 60). Results: The total α-syn and tau concentrations as well as α-syn/tau heterodimers were significantly lower in the LBD group and the AD group compared with HC, whereas α-syn/Aβ1–42 concentrations were significantly lower in the AD dementia group only. RBC α-syn/tau heterodimers had a higher diagnostic accuracy for differentiating patients with LBD versus HC (AUROC = 0.80). Conclusion: RBC α-syn heteromers may be useful for differentiating between neurodegenerative dementias (LBD and AD) and HC. In particular, RBC α-syn/tau heterodimers have demonstrated good diagnostic accuracy for differentiating LBD from HC. However, they are not consistently different between LBD and AD. Our findings also suggest that α-syn, Aβ1–42, and tau interact in vivo to promote the aggregation and accumulation of each other.