Treatment with olanzapine depot is associated with a rare but potentially adverse reaction, namely post-injection delirium/sedation syndrome (PDSS), characterized by delirium and/or sedation. This is a case report of a 38-year-old male patient who developed symptoms consistent with PDSS shortly after receiving intramuscular injection of olanzapine depot. Clinicians should be aware of PDSS and observe patients for three hours after receiving the injection, measuring vitals and referring to medical care if necessary.