NMDA receptors have been implicated in the pathophysiological mechanisms of fear memory and impairment of fear memory extinction. The purpose of this study was to investigate the effects of dextromethorphan (DXM), an NMDA receptor antagonist, on fear memory extinction in a rat model of posttraumatic stress disorder (PTSD). Using isoflurane instead of diethyl ether, we established a rat model subjected to the Single Prolonged Stress (SPS) protocol and evaluated the effects of DXM on fear memory extinction. Rats were conditioned with fear using a conditioned stimulus (CS: chamber) and an unconditioned stimulus (US: foot shock), and their freezing time was measured for three days upon re-exposure to the chamber. The group receiving long-term DXM treatment (40 mg/kg, i.p.) after SPS loading showed no improvement in freezing time. However, the group that received a single administration of DXM (40 mg/kg, i.p.) during the experimental fear memory extinction process of SPS-loaded rats showed reduced freezing behavior. Interestingly, DXM did not affect fear memory retrieval or consolidation, suggesting its specificity for facilitating fear memory extinction. These findings suggest DXM as a potential therapeutic agent for promoting fear memory extinction in the context of PTSD symptomatology, providing insight into novel pharmacological interventions for this debilitating disorder.