Objective: The purpose of this study was to evaluate early changes in myocardial function in overweight and obese children without hypertension. Methods: This was a cross-sectional study involving 150 participants of both sexes between 6 and 15 years old. Anthropometric and biochemical evaluations were performed. Ventricular function was assessed by conventional echocardiographic methods and myocardial deformation analysis by two-dimensional speckle tracking echocardiography. One-way analysis of variance was employed for the global comparison of study variables between groups (children with normal weight, overweight, and obesity), and post hoc analysis with Bonferroni correction was used for multiple comparisons, considering normal weight children as the reference category. Results: Overall, 142 participants were included, 50 (35%) with normal weight, 39 (28%) overweight, and 53 (37%) obesity. The diastolic diameter of the left ventricular and interventricular septum, and diameter of the left atrium and LV mass were significantly higher in children with obesity compared to those with normal weight. No significant differences in the conventional indicators of LV systolic and diastolic function were found between groups. Significant differences in the regional myocardial deformation between the three groups were observed. Mean global longitudinal myocardial deformation was smaller in patients with obesity (−20.9% vs. −23.5%, p < 0.05) compared to children with normal weight. Conclusions: The childhood obesity was associated with altered myocardial deformation, even in the presence of normal ejection fraction. Myocardial deformation evaluation is relevant in the assessment of pediatric patients with obesity.