Purpose: To establish a cut-off value of main pulmonary artery (MPA)/descending aorta (DA) ratio and MPA/ascending aorta (AA) ratio by computed tomography (CT) to identify pulmonary hypertension (PHT) in children. Material and methods: A total of 45 children diagnosed with PHT, who underwent both right heart catheterization (RHC) and CT, were enrolled as a case group (PHT), and their CT findings were compared with those from a control group (non-PHT). The widest short-axis diameters of DA at the level of the diaphragmatic outlet, AA, and MPA at its bifurcated level were measured. Results: The most common indication to order a CT in the PHT group was congenital heart disease (CHD) (n = 30, 66.7%), and in the non-PHT group it was metastatic workup (n = 31, 68.9%). The median (IQR) diameters of MPA in the PHT and the non-PHT groups were 18.7 mm (15.5, 26.7) and 16.4 mm (13.7, 19.5) (p = 0.005). The MPA/DA ratios were 2.2 and 1.5 in the PHT and non-PHT groups (p < 0.001). The MPA/AA ratios were 1.2 and 1.1 in the PHT and non-PHT groups (p = 0.042). An MPA/DA ratio of 1.8 carried a positive likelihood ratio (LR+) of 7.5 with a sensitivity of 66.67%, specificity of 91%, positive predictive value (PPV) of 88%, and negative predictive value (NPV) of 73.21%. Conclusions: The MPA/DA ratio > 1.8 suggests PHT in children and may lead to the avoidance of invasive cardiac catheterization particularly in non-CHD patients.