Background. Alterations in plantar soft tissues are often reported in adults with diabetes, whereas data on children are conflicting. Also, the extent of foot damage caused by excess body fat in children has not been fully characterized yet. This study aimed to address the relationship between body mass and structural changes of the foot in children and adolescents with and without diabetes. Methods. In a case-control study, 43 participants (age 13 ± 2.6 years) were recruited, 29 (67%) with type 1 diabetes (T1D) and 14 (33%) controls. Anthropometric parameters [body mass index (BMI), waist circumference (WC), and waist-to-height ratio (WHtR)], foot posture index-6 (FPI-6) for static foot posture, and navicular drop test (NDT) for medial longitudinal arch height (MLA) were measured in all participants. The thickness of the midfoot plantar fascia (MPF) and medial midfoot fat pad (MMFP) were quantified using ultrasound. Results. No differences in clinical and ultrasonographical parameters were observed between the study groups. MMFP thickness was correlated with MPF thickness (p = 0.027). MMFP and MPF thicknesses were positively associated with BMI (p < 0.001 and p = 0.013, respectively), WC (p < 0.001 and p = 0.013), and WHtR (p < 0.001 and p = 0.026). The NDT measured on the right and left foot correlated with WHtR (p = 0.038 and p = 0.009, respectively), but not with WC and BMI. Conclusions. Children with T1D show structural alterations of plantar soft tissues which seem related to body mass increase rather than diabetes pathology. Ultrasound is a valuable tool to assess early structural changes of the foot in young people with an elevated BMI.