Background Renal artery stenosis is one of the secondary causes of hypertension in children. Hyponatremic hypertensive syndrome (HHS) is an uncommon disorder usually seen in adults with unilateral renal artery stenosis and atherosclerosis but it’s rarely reported in children. Case presentation A 5-year-old male child presented with generalized tonic clonic convulsions associated with severe hypertension, hyponatremia, hypokalemia, and polyuria. Laboratory tests revealed elevated plasma renin activity, and aldosterone level. The diagnosis of hyponatremic hypertensive syndrome secondary to unilateral renal artery stenosis was made. MRI of the brain was consistent with posterior reversible leukoencephalopathy syndrome (PRES). The patient responded to left renal artery stenting with control of blood pressure. Conclusions Findings of hyponatremia, hypokalemia, hypertension, polyuria can be attributed to unilateral renal artery stenosis with associated HHS. Early recognition is important for early management of such treatable condition to prevent irreversible renal damage.