Background: The COVID-19 pandemic has been associated with rising obesity rates. Autistic children have a higher risk of obesity than neurotypical children. Our study aims to describe the changes in overweight/obesity rates in autistic children during the pandemic, and to identify contributing factors. Methods: This is a retrospective case record review of patients with a clinical diagnosis of autism, who were seen at a developmental-behavioral pediatrics clinic in a tertiary academic hospital, between 1 January 2019 and 24 October 2021. We compared the average monthly rates of overweight/obese status pre- and during the pandemic. We collected data on the patients’ and parents’ demographics, duration of screen time per day, degree of difficulties related to autism symptoms and cognition. We analyzed factors associated with being overweight/obese during the pandemic. Results: 1330 patient visits were included. The mean age was 45.4 months; 78% were male; 52% were Chinese. The average monthly rate of overweight/obese status increased by 1.8% during the pandemic (17.9% pre-pandemic; 19.7% during pandemic). Factors associated with being overweight/obese during the pandemic included: Malay ethnicity (OR 2.321, p < 0.01), developmental delay (OR 2.80, p < 0.01), and lower parental education level (father OR 1.73, p = 0.01; mother OR 1.63, p = 0.03). On multivariate analysis, only Malay ethnicity (OR 2.95, p = 0.01) was significant. Conclusions: Our study demonstrates a rising overweight/obesity rate amongst children with autism spectrum disorder during the pandemic. It also identified higher-risk patient profiles (Malay race, developmental delay, lower parental education). We hope this will facilitate the implementation of preventative health measures specifically supporting the high-risk children.