Paediatric pulmonary arterial hypertension (PAH) after neonatal arterial switch operation (ASO) for transposition of the great arteries (TGA) is a clinically recognised entity with an estimated incidence of 0.6%-1.0%. Nevertheless, a clinical characterisation is lacking. We present an international cohort of children with PAH after neonatal ASO for TGA and describe epidemiology and clinical course. Data were collected of children with PAH after neonatal ASO (≤6 weeks after birth) for simple TGA without residual shunt defects, identified in four national paediatric PAH networks in Europe and one US referral centre. Twenty-five children were identified between 1989 and 2014. In 17 children (68%), PAH was detected <1 year after ASO. In the remaining children, PAH was detected after median 64 months (IQR 19.5, 94.5). Nineteen children (96%) received PAH-targeted therapies. During follow-up after ASO (median 5.2 years), eight children died, four underwent lung transplantation and two received a Potts shunt. 1-year and 5-year Potts shunt- and transplantation-free survival after ASO was 100% and 73%. From first PAH detection, this was 100% and 58%, respectively, which did not differ between children with early (<1 year after ASO) or late PAH detection. The occurrence of PAH after ASO for TGA represents a specific association. PAH onset may be early or late after ASO, with similar fatal course from first PAH detection. Mechanisms leading to PAH in this association are unknown, but may include abnormal prenatal pulmonary haemodynamics and/or genetic susceptibility. Routine, lifelong follow-up for children who undergo ASO for TGA should include screening for PAH.