Polysialic acid (PSA), a highly negatively charged glycan attached mainly to the neural cell adhesion molecule (NCAM), is emerging as a critical but underrecognized extracellular regulator of glutamatergic neurotransmission. While previous literature has focused on PSA’s developmental roles, increasing evidence indicates that PSA–NCAM also contributes to synaptic plasticity mechanisms in the mature brain. This review integrates evidence from structural biophysics, single-channel electrophysiology, and disease models to explain how PSA modulates glutamate receptor gating to control learning and memory. We synthesize findings from biochemical reconstitution, electrophysiological recordings, and in vivo studies to show that PSA can modulate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor open probability, burst duration, and cooperative gating without affecting conductance, thereby promoting long-term potentiation. Conversely, PSA selectively suppresses GluN2B-containing extrasynaptic N-methyl D-Aspartate (NMDA) receptor activity by lowering open probability and calcium influx, maintaining an optimal balance between potentiation and depression while providing neuroprotection. Disruption of PSA–NCAM signaling in developmental and disease models, including prenatal cannabinoid exposure and neurodegeneration, produces cognitive deficits reversible by PSA restoration. Notably, much of the current evidence derives from in vitro systems, with relatively few studies conducted in vivo, and studies employing PSA mimetics mostly, which should be considered when interpreting physiological relevance. Collectively, the available evidence suggests that PSA functions as an extracellular modulator linking synaptic glycans to glutamate receptor regulation and plasticity related signaling pathways, highlighting the potential importance of extracellular glycan mechanisms in the control of synaptic function.