Weight status on children and youth with autism spectrum disorder is limited. We examined the prevalence of overweight/obesity in children and youth with autism spectrum disorder, and associations between weight status and range of factors. Children and youth with autism spectrum disorder aged 2–16 years (n = 208) and their parents participated in this study. Body mass index was calculated using the Centers for Disease Control and Prevention growth charts and the International Obesity Task Force body mass index cut-offs. The Autism Diagnostic Observation Schedule was administered. Parents completed questionnaires about socio-demographics, diagnosed comorbidities, sleep disturbances, social functioning and medication of youth with autism spectrum disorder. The prevalence of overweight/obesity in participants with autism spectrum disorder was 35%. One quarter of obese children and youth (25.6%) had obese parents. There was a significant association between children and youth’s body mass index and maternal body mass index (r = 0.25, n = 199, p < 0.001). The gender and age, parental education, family income, ethnicity, autism spectrum disorder severity, social functioning, psychotropic and complementary medication use of children and youth with autism spectrum disorder were not statistically associated with their weight status. Findings suggest the need for clinical settings to monitor weight status of children and youth with autism spectrum disorder in a bid to manage or prevent overweight/obesity in this population. Incorporating a family system approach to influence health behaviours among children and youth with autism spectrum disorder especially for specific weight interventions is warranted and should be further explored.