Background: The increase in the survival of patients with D-Transposition of the great arteries (TGA) after arterial switch operation (ASO) has now turned our focus to the evaluation of mid and long-term outcomes. Although most patients are followed by conventional echocardiography, the study of cardiac functionality and morphometric parameters in children with TGA after ASO is scarce. The present study aims to describe the functional and morphometric echocardiographic changes in children after ASO. Methods: We performed an observational study in patients aged 1–5 years with TGA who underwent neonatal ASO. Morphometric and functional echocardiographic parameters were analyzed in 21 patients and compared with 52 age-matched healthy controls. Results: We found morphological and functional changes, especially in the right ventricle, which is more globular (right ventricle [RV] basal sphericity index 1.5 vs. 1.8, P = 0.016), and with a decreased systolic function compared to healthy controls (fractional area change 51 vs. 58%, P = 0.006; tricuspid annular plane systolic excursion 13 vs. 20 mm, P = 0.001; s’ 7 vs. 12 cm/s, P = 0.001). In the speckle-tracking strain imaging, there was a decrease in the longitudinal deformation of the apical septal myocardium (−23% vs. −27%; P = 0.005). Preoperative systemic overload to the right ventricle could be an important factor in the origin of these changes. Conclusions: In patients with TGA after ASO, there are morphometric and functional echocardiographic changes, such as globular form and decreased function, especially in the RV; the effect of these changes on long-term outcomes would require prospective follow-up studies.