--- - |2+ Abstract Aim Loss‐of‐function mutation of Akt3 in humans has been associated with microcephaly and cognitive defects. Two Akt isoforms, Akt1 and Akt3, are highly expressed in hippocampal pyramidal cells. We explored the roles of Akt1 and Akt3, respectively, in spatial cognition and underlying mechanisms. Methods We used Akt1 knockout (Akt1‐KO) and Akt3 knockout (Akt3‐KO) mice to examine the influence of Akt1 and Akt3 deficiency on spatial memory, as well as induction and maintenance of hippocampal CA1 NMDA receptor‐dependent and protein synthesis‐dependent long‐term potentiation (LTP). Results Long‐term spatial memory was impaired in Akt3‐KO mice, but not in Akt1‐KO mice, as assessed by the Morris water maze task. Akt3‐KO and Akt1‐KO mice displayed reductions in brain size without concurrent changes in the number of pyramidal cells or basal properties of synaptic transmission. One‐train high‐frequency stimulation (HFS × 1) induced NMDA receptor‐dependent LTP in Akt3‐KO mice and Akt1‐KO mice. Four‐train HFS (HFS × 4) induced rapamycin‐sensitive long‐LTP in Akt1‐KO mice, but not Akt3‐KO mice. Basal level of mTOR phosphorylation was reduced in Akt3‐KO mice rather than Akt1‐KO mice. HFS × 4 induced an elevation of mTOR and p70S6K phosphorylation in Akt1‐KO mice, which led to enhanced 4EBP2 and eIF4E phosphorylation along with an increase in AMPA receptor protein. However, the same protocol of HFS × 4 failed to trigger the mTOR‐p70S6K signalling cascade or increase 4EBP2 and eIF4E phosphorylation in Akt3‐KO mice. Conclusion The Akt3 deficiency via inactivation of mTOR suppresses HFS × 4‐induced mTOR‐p70S6K signalling to reduce phosphorylation of 4EBP and eIF4E, which impairs protein synthesis‐dependent long‐LTP and long‐term spatial cognitive function. - 'Acta Physiologica, EarlyView. '