Background: Antipsychotic-induced weight gain (AIWG) is a common and clinically significant side effect of many second-generation antipsychotics. While lurasidone is generally associated with a more favourable metabolic profile, individual variability in response remains a concern. Real-world evidence is still limited regarding its long-term metabolic side effects. Case Report: A 44-year-old woman with a long-standing history of psychotic symptoms, including persecutory delusions, who had previously been stabilized on long-acting injectable paliperidone. She developed hyperprolactinemia and amenorrhea after several doses, prompting a switch to oral lurasidone (40 mg daily). While her menstrual cycle normalized post switch, she experienced significant weight gain of approximately 15 kg over six months, despite no changes in lifestyle or concurrent medications. Conclusion: This case highlights that, although lurasidone is generally considered metabolically neutral, significant weight gain can still occur in certain individuals. Clinicians should remain vigilant for metabolic side effects even when prescribing antipsychotics perceived as lower risk, and personalized monitoring remains essential in long-term management.