We describe a female infant with incontinentia pigmenti complicated by severe pulmonary arterial hypertension that was markedly improved by tadalafil administration. The infant was referred to our institution because of neonatal seizures and generalized skin rash at the age of 1 day. She was diagnosed with incontinentia pigmenti on skin biopsy findings. In addition to incontinentia pigmenti, she had pulmonary arterial hypertension without structural heart disease. The pulmonary hypertension rapidly worsened at the age of 2 months and was confirmed by cardiac catheterization. The pulmonary artery pressure was equal to systemic pressure but it decreased in response to nitric oxide inhalation. We, therefore, initiated treatment with tadalafil of 1 mg/kg/day. The follow-up cardiac catheterization performed at 9 months revealed dramatic improvement in the pulmonary artery pressure. An IKBKG mutation with deletion of exons 4-10 was detected in the blood of both the patient and her mother. Our experience indicates that tadalafil may be beneficial in treating pulmonary arterial hypertension associated with incontinentia pigmenti.