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The neural cell adhesion molecule NCAM and its association with the polysialic acid (PSA) are believed to contribute to brain structural plasticity that underlies memory formation. Indeed, the attachment of long chains of PSA to the glycoprotein NCAM down-regulates its adhesive properties by altering cell-cell interactions. In the brain, the biosynthesis of PSA is catalyzed by two polysialyltransferases, which are differentially regulated during lifespan. One of them, ST8SiaIV (PST), is predominantly expressed during adulthood whereas the other one, ST8SiaII (STX), dominates during embryonic and post-natal development. To understand the role played by ST8SiaIV during learning and memory and its underlying hippocampal plasticity, we used knockout mice deleted for the enzyme ST8SiaIV (PST-ko mice). At adult age, PST-ko mice show a drastic reduction of PSA-NCAM expression in the hippocampus and intact hippocampal adult neurogenesis. We found that these mice display impaired long-term but not short-term memory in both, spatial and non-spatial behavioral tasks. Remarkably, memory deficits of PST-ko mice were abolished by exposure to environmental enrichment that was also associated with an increased number of PSA-NCAM expressing new neurons in the dentate gyrus of these mice. Whether the presence of a larger pool of immature, likely plastic, new neurons favored the rescue of long-term memory in PST-ko mice remains to be determined. Our findings add new evidence to the role played by PSA in memory consolidation. They also suggest that PSA synthesized by PST critically controls the tempo of new neurons maturation in the adult hippocampus.