Hyperphosphorylation of tau protein (tau) causes neurodegenerative diseases such as Alzheimer's disease (AD). Recent studies of the physiological correlation between tau and α–synuclein (α–SN) have demonstrated that: (a) phosphorylated tau is also present in Lewy bodies, which are cytoplasmic inclusions formed by abnormal aggregation of α–SN; and (b) the neurotoxin 1–methyl–4–phenyl–1,2,3,6–tetrahydropyridine (MPTP) increases the phosphorylation of tau as well as the protein level of α–SN in cultured neuronal cells, and also in mice. However, the molecular mechanism responsible for the α–SN–mediated hyperphosphorylation of tau remains to be elucidated. In this in vitro study, we found that: (a) α–SN directly stimulates the phosphorylation of tau by glycogen synthase kinase–3β (GSK–3β), (b) α–SN forms a heterotrimeric complex with tau and GSK–3β, and (c) the nonamyloid beta component (NAC) domain and an acidic region of α–SN are responsible for the stimulation of GSK–3β–mediated tau phosphorylation. Thus, it is concluded that α–SN functions as a connecting mediator for tau and GSK–3β, resulting in GSK–3β–mediated tau phosphorylation. Because the expression of α–SN is promoted by oxidative stress, the accumulation of α–SN induced by such stress may directly induce the hyperphosphorylation of tau by GSK–3β. Furthermore, we found that heat shock protein 70 (Hsp70) suppresses the α–SN–induced phosphorylation of tau by GSK–3β through its direct binding to α–SN, suggesting that Hsp70 acts as a physiological suppressor of α–SN–mediated tau hyperphosphorylation. These results suggest that the cellular level of Hsp70 may be a novel therapeutic target to counteract α–SN–mediated tau phosphorylation in the initial stage of neurodegenerative disease.