Paraneoplastic syndrome (PNS) related to lung cancer is very common. However, the syndrome of inappropriate anti-diuretic hormone secretion (SIADH) is rare in non-small cell lung cancer (NSCLC). We are reporting the case of a 58-year-old female presenting with dyspnea, cough, weight loss, digital clubbing, and one week of haemoptysis. CT showed a mediastinal mass completely encasing her superior vena cava, causing significant narrowing of the trachea and right mainstem bronchus. Bronchoscopy and biopsy identified a non-resectable NSCLC. Palliative radiation therapy was initiated. The day after her first radiation treatment, the patient developed asymptomatic hyponatremia, confirmed to be SIADH by laboratory evaluation. NSCLC-associated SIADH has been reported only thrice over the past two decades and never following radiation therapy with clinical improvement. The patient was discharged home on fluid restriction after her respiratory status improved to continue outpatient radiation and chemotherapy. SIADH in the setting of NSCLC is discussed.