Obesity is considered the main risk factor associated with the development of insulin resistance (IR). The aim of this study was to evaluate the influence of obesity on puberty onset and IR in Mexican children. A total of 378 children (189 boys and 189 girls) aged 8–14 years participated in the study. IR was estimated using the homeostasis model assessment for IR (HOMA-IR). The mean fasting glucose (FG) and basal insulin levels were 82 mg/dl and 11.0 μIU/ml in boys and 77.3 mg/dl and 12.3 μIU/ml in girls (P < 0.05). Subjects with obesity at Tanner stages II–V showed increased FG levels (P < 0.05). In boys with obesity, there was a decrease in HOMA-IR in Tanner stage IV and differences in age between boys with normal weight and those with obesity in Tanner V, being older the boys with obesity. Obesity in children and adolescents was associated with higher HOMA-IR values. In boys with obesity, IR increased at the end of pubertal maturation, with a delay in puberty. These findings should be considered on the establishment of IR cutoff values for pubertal population in Mexico and in the establishment of strategies to prevent the health problems related to obesity.