Background and Purpose Growing evidence demonstrates that the delta opioid receptor (DOP) is an attractive candidate for novel antidepressants with the potential to exhibit rapid action with few adverse effects. However, the underlying detailed functional mechanism remains elusive. Previously, we reported that the selective DOP agonist, KNT-127, produced robust antidepressant-like effects in the mice forced swimming test (FST). Thus, we attempted to identify the cellular mechanism underlying this effect. Experimental Approach Male ICR mice (4–6 weeks) were used in all experiments. The FST was conducted as a screening model for antidepressants. The phosphorylation level of proteins in specific brain regions was quantified using Western blotting. Glutamate/gamma-aminobutyric acid-dependent postsynaptic currents were detected using whole-cell voltage-clamp recordings. Key Results The selective mTOR inhibitor, rapamycin, and the PI3K inhibitor, LY294002, blocked the antidepressant-like effects of KNT-127 in the FST. KNT-127 increased the phosphorylation level of mTOR signal-related proteins, Akt and p70S6K, in the medial prefrontal cortex. The bilateral microinfusion of KNT-127 in the infralimbic cortex decreased immobility in the FST. The frequency of miniature excitatory postsynaptic currents in the infralimbic cortex increased and that of miniature inhibitory postsynaptic currents decreased with the perfusion of KNT-127, which was blocked by pretreatment with rapamycin. Conclusions and Implications KNT-127 displays antidepressant-like actions through the direct facilitation of neuronal excitability in the mice infralimbic cortex, which is implicated in the PI3K-Akt-mTOR-p70S6K signaling pathway. These results could indicate the first steps in elucidating the complete mechanical functions of DOPs as a potential candidate for novel antidepressants.