In this report, we present a five-month-old female patient with a diagnosis of idiopathic pulmonary arterial hypertension (IPAH). This patient was accepted by our clinic after a sudden deterioration in her clinical condition. We performed echocardiography and observed a 17 mm pericardial fluid near the left ventricle posterior wall, which was seen to compress the right atrium and right ventricle outflow tract. Cardiac contraction was impaired, the interventricular septum was hypertrophic and the right ventricle systolic pressure measured by tricuspid regurgitation was at systemic level. Pericardiocentesis was performed for pericardial tamponade, and a drainage tube was placed in the pericardium. Cardiac catheterization was performed before and after nitric oxide inhalation, and there were no changes in the parameters of pulmonary vascular resistance, pulmonary vascular resistance/systemic vascular resistance ratio and mean pulmonary arterial pressure. No cardiac lesion that could cause PAH was detected during catheterization. Under mechanical ventilatory support and intensive medical therapy, the patient died on her 16th day of hospitalization. The importance of our case is that it is the first infantile case presenting with pericardial tamponade associated with IPAH.