Background Patients with cancer often develop syndrome of inappropriate antidiuretic hormone (SIADH), most commonly caused by paraneoplastic syndrome. However, chemotherapeutic agents can also induce SIADH. Although rare, cisplatin-induced SIADH has been reported in patients with various cancers. Hyponatremia, a consequence of SIADH, can cause seizures, coma, and even death. Case Presentation A 76-year-old female with FIGO stage IIIa cervical cancer was scheduled for definitive CCRT, including low-dose cisplatin (40 mg/m2 weekly). She previously had rectal cancer, treated surgically two years prior, followed by FOLFOX chemotherapy. Two days after her third cisplatin session, she presented with drowsiness but was hemodynamically stable. She was initially admitted to a general ward and soon transferred to the ICU for closer monitoring and urgent management. She was euvolemic, and laboratory tests revealed a serum sodium of 114 mmol/L. Serum osmolality was 246 mOsm/kg, urine osmolality 648 mOsm/kg, and urine sodium level 122 mmol/L, clinically consistent with the diagnosis of SIADH. Table 1 summarizes the changes in laboratory findings. Free water intake was restricted to a maximum of 700 mL per day and sodium was supplemented by intravenous hypertonic saline using a rapid intermittent bolus regimen. Serum sodium correction was carefully controlled to prevent overcorrection, ensuring an increase of no more than 8 mmol/L per 24 hours. On hospital day 4, after serum sodium normalized to 136 mmol/L, all intravenous fluids were discontinued, and oral diet was resumed. Her serum sodium remained within normal range until discharge on hospital day 8. Discussion This case describes SIADH-induced severe hyponatremia after low-dose cisplatin administration. Cisplatin-induced SIADH is uncommon, and its underlying mechanism remains unclear. Given the widespread use of cisplatin, prompt recognition and management of SIADH and hyponatremia are crucial.