Opioid-associated amnestic syndrome (OAS) is a relatively new condition that is associated with opioid abuse and has increased in prevalence since the notable rise in opioid-related deaths and opioid-related hospitalizations of the opioid crisis. Patients often present with acute anterograde amnesia and current opioid abuse, most commonly fentanyl. OAS is frequently diagnosed when other potentially infectious or metabolic conditions such as encephalitis and seizures are ruled out, as these conditions can also present similarly to OAS. This case highlights the signs, symptoms, and hallmark characteristics of OAS, including bilateral hippocampal edema and anterograde amnesia.