The weight of synaptic connections, which is controlled not only postsynaptically but also presynaptically, is a key determinant in neuronal network dynamics. The mechanisms controlling synaptic weight, especially on the presynaptic side, remain elusive. Using single-synapse imaging of the neurotransmitter glutamate combined with super-resolution imaging of presynaptic proteins, we identify a presynaptic mechanism for setting weight in central glutamatergic synapses. In the presynaptic terminal, Munc13-1 molecules form multiple and discrete supramolecular self-assemblies that serve as independent vesicular release sites by recruiting syntaxin-1, a soluble N-ethylmaleimide-sensitive-factor attachment receptor (SNARE) protein essential for synaptic vesicle exocytosis. The multiplicity of these Munc13-1 assemblies affords multiple stable states conferring presynaptic weight, potentially encoding several bits of information at individual synapses. Supramolecular assembling enables a stable synaptic weight, which confers robustness of synaptic computation on neuronal circuits and may be a general mechanism by which biological processes operate despite the presence of molecular noise.