We report a case of a premature newborn baby who presented with hyponatremia, hyperkalemia, and metabolic acidosis accompanied by severe polyhydramnios in utero. The baby was diagnosed with pseudohypoaldosteronism on the basis of normal 17-hydroxyprogesterone levels, elevated aldosterone, and clinical symptoms. His serum electrolyte levels were corrected with sodium chloride supplementation. Sodium supplementation was reduced gradually and discontinued at 5 months of age. At 5 months, the child was able to maintain normal serum electrolyte levels without oral sodium chloride supplementation, and showed normal physical and neurological development. This case illustrates that pseudohypoaldosteronism must be considered if a newborn infant with an antenatal history of severe polyhydramnios shows excessive salt loss with normal levels of 17-hydroxyprogesterone.