Metoclopramide and domperidone are dopamine antagonists that can cause an acute dystonic reaction. Metoclopramide is a rare but major contributor to serotonin syndrome, particularly when used with other serotonergic agents. Serotonin syndrome is a rare, potentially life-threatening adverse reaction characterized by a triad of altered mental status, autonomic dysfunction, and neuromuscular hyperactivity that typically results from exposure to serotonergic agents. Herein, we report a previously healthy 9-year-old girl who was brought to the emergency department with an alteration in the level of conscious and involuntary repetitive movements after approximately 24 hours of receiving a therapeutic dose of metoclopramide and domperidone. Physical examination showed tachycardia, hyperthermia, and a Glasgow Coma Scale score of 11, as well as mydriasis and hyperreflexia. In addition to resolving the symptoms after administering midazolam and diphenhydramine, the diagnosis of serotonin syndrome was made based on the classical symptoms and signs, which met the Hunter criteria. This case indicates the need for clinical awareness of the life-threatening syndrome and caution with medications having potential interactions with metoclopramide.