In Parkinson disease (PD), α-synuclein aggregates called Lewy bodies often involve and sequester Septin4 (Sept4), a polymerizing scaffold protein. However, the pathophysiological significance of this phenomenon is unclear. Here, we show the physiological association of Sept4 with α-synuclein, the dopamine transporter, and other presynaptic proteins in dopaminergic neurons; mice lacking Sept4 exhibit diminished dopaminergic neurotransmission due to scarcity of these presynaptic proteins. These data demonstrate an important role for septin scaffolds in the brain. In transgenic mice that express human α-synuclein^A53T (a mutant protein responsible for familial PD), loss of Sept4 significantly enhances neuropathology and locomotor deterioration. In this PD model, insoluble deposits of Ser¹²⁹-phosphorylated α-synuclein^A53T are negatively correlated with the dosage of Sept4. In vitro, direct association with Sept4 protects α-synuclein against self-aggregation and Ser¹²⁹ phosphorylation. Taken together, these data show that Sept4 may be involved in PD as a dual susceptibility factor, as its insufficiency can diminish dopaminergic neurotransmission and enhance α-synuclein neurotoxicity.