Background: Obesity in children and adolescents in the 21st century has become a major issue in the global health field. Previous studies showed that obese children have substantial adverse short- and long-term health consequences, including asthma, chronic low-grade systemic inflammation, diabetes, and numerous orthopedic complications. The purpose of this study was to investigate the relationship between body mass index (BMI) and the development of gross motor skills in 5-7-year-old children. Methods: This was a cross-sectional study conducted on children 5 - 7 years old in Bojnourd, Iran. The sampling method was multistage. A checklist containing the demographic data and BMI of participants was completed. Ulrich’s test of gross motor development (3rd edition) was also used to assess gross motor skills in children. Results: The age range of participants was 5 - 7 years. The mean ± standard deviation (SD) of the participants’ age was 6.24 ± 0.82 years. Furthermore, the participants’ mean ± SD weight was 24.8 ± 3.68 g (range: 17.5 - 32.2 g). There was a significant statistical difference in gross motor skills scores in the three groups of obese, overweight, and optimal weight (P < 0.0001). There was also a significant difference in gross motor skills scores between the two gender groups (P < 0.0001). Conclusions: Higher weight groups had lower motor skills scores. Therefore, it is necessary to adopt appropriate policies for physical activity and to reduce the prevalence of obesity and overweight in children.