Background Patients with transposition of the great arteries (TGA) have an altered aortic geometry after an arterial switch operation (ASO), with neo‐aortic root dilatation as an important complication. Geometry‐related aortic hemodynamics have been assumed to contribute to pathology of the ascending aorta (AAo). Purpose To evaluate aortic flow displacement (FD) and regional wall shear stress (WSS) in relation to ascending neo‐aortic geometry in children after ASO. Study Type Prospective. Population Twenty‐eight TGA patients after ASO and 10 healthy volunteers. Field Strength/Sequence 3.0T/4D flow (segmented fast‐spoiled echo pulse), noncontrast‐enhanced MR angiography (Dixon), and anatomic images (SSFP). Assessment Aortic diameters and body surface area‐indexed aortic dimensions (Z‐scores), normalized FD and planar ascending aortic WSS. Statistical Tests Mann–Whitney and chi‐square tests for differences in FD magnitude, WSS, and FD directionality between groups, respectively. Spearman rank correlation to assess the degree of association between aortic geometry, FD and WSS parameters. Shapiro–Wilk test to evaluate distribution normality on the absolute differences in octant location between FD and WSS. Results TGA patients showed a significantly dilated proximal AAo and relatively small mid‐AAo dimensions at the level of the pulmonary arteries (Z‐scores neo‐aortic root: 4.38 ± 1.96 vs. 1.52 ± 0.70, P < 0.001; sinotubular junction: 3.48 ± 2.67 vs. 1.38 ± 1.30, P = 0.010; mid‐AAo: 0.32 ± 3.06 vs. 1.69 ± 1.24, P = 0.001). FD magnitude was higher in TGA patients (neo‐aortic root: 0.048 ± 0.027 vs. 0.021 ± 0.006, P < 0.001; sinotubular junction: 0.054 ± 0.037 vs. 0.029 ± 0.013, P < 0.05) and was related to the neo‐aortic Z‐score. Clear areas of higher WSS at the right and anterior aortic wall regions along the distal AAo were detected in TGA patients, most pronounced in those with relatively smaller mid‐AAo diameters. Data Conclusion TGA‐specific geometry related to the ASO, evidenced by neo‐aortic root dilatation and a sudden change in vessel diameter at mid‐AAo level, leads to more aortic flow asymmetry in the proximal AAo and WSS distribution with higher WSS at the right and anterior aortic wall regions along the distal AAo. Level of Evidence: 1 Technical Efficacy Stage: 3 J. Magn. Reson. Imaging 2020;51:1105–1116.