Hyponatraemia, defined as serum sodium concentration below 135 mmol/l, is significantly common in patients with malignancies. It usually causes mild neurological and cardiovascular symptoms but can also be a life-threatening condition. Hyponatraemia is a risk factor for bad prognosis. It affects lower survival time and worse response to anticancer treatment. It may be due to the tumour, anticancer treatment, or comorbidities. The most common cause of hyponatraemia in patients with malignancies is a syndrome of inappropriate antidiuretic hormone secretion. After diagnosing hyponatraemia, the duration and severity of symptoms should be determined. Acute and life-threatening hyponatraemia requires immediate treatment with 3% saline. Chronic hyponatraemia treatment targets the cause.