Polysialic acid (PSA) regulates functions of the neural cell adhesion molecule (NCAM) during development and in neuroplasticity in the adult; the underlying mechanisms at different phases of learning and memory consolidation are, however, unknown. To investigate the contributions of PSA versus the extracellular domain of the NCAM glycoprotein backbone to synaptic plasticity, we applied NCAM, PSA-NCAM, and PSA to acute slices of the hippocampal CA1 region of NCAM-deficient mice and measured their effects on long-term potentiation (LTP). Remarkably, only PSA and PSA-NCAM, but not NCAM restored normal LTP. Application of these molecules to the dorsal hippocampus of wild-type mice showed that PSA-NCAM and PSA, but not NCAM, injected before fear conditioning, impaired formation of hippocampus-dependent contextual memory. Consolidation of contextual memory was affected by PSA-NCAM only when injected during its late, but not early phases. None of the tested compounds disturbed extrahippocampal-cued memory. Mice lacking the polysialyltransferase (ST8SialV/PST) responsible for attachment of PSA to NCAM in adulthood showed a mild deficit only in hippocampal contextual learning, when compared with NCAM-deficient mice that were disturbed in both contextual and cued memories. Contextual and tone memory in NCAM-deficient mice could be partially restored by injection of PSA-NCAM, but not of NCAM, into the hippocampus, suggesting that the impact of PSA-NCAM in synaptic plasticity and learning is not mediated by modulation of NCAM–NCAM homophilic interactions. In conclusion, our data support the view that polysialylated NCAM is involved in both formation and late consolidation of contextual memory.