The mechanism underling dysfunction of cellular proteostasis on α-synuclein (α-Syn) leading to pathogenesis of synucleinopathy remains unclear. Recently, we reported that the binding of an RNA secondary structure G-quadruplex (G4RNA) to a prion-like protein FMRpolyG causes its liquid-to-solid phase transition, leading to neurodegeneration in a hereditary neurodegenerative disease, Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) (Sci Adv. 2021). Here, we introduce the possibility that G4RNA is a key pathogen on the phase transition of α-Syn. Purified α-Syn protein binds to G4 structure formed RNA specifically, and that the addition of G4RNA promoted the liquid-to-solid phase transition on α-Syn under molecular crowding in vitro. In mouse primary neurons, G4RNA assembly was immediately observed under cellular stress conditions, thereafter co-aggregation of α-Syn with G4RNA was occurred. Artificial assembly of G4RNA using an optogenetic approach initiated α-Syn aggregation, thereby elicits neuronal dysfunction in mouse primary neurons. These results suggest that G4RNA assembly evoked by various cellular stress triggers to develop aggregation of α-Syn, which may be a cellular mechanism underlying onset of sporadic synucleinopathy. We now analyze relationship between G4RNA and α-Syn aggregation in vivo.