Background Overweight and obesity are common in adults with intellectual disabilities, which complicates their health. To meet their health needs, individuals with intellectual disability frequently rely on proxies to answer questions on their behalf. In the general population, the use of proxy‐reported height and weight to compute body mass index (BMI) has been validated, but not among adults with intellectual disability. The objective of this study was to determine the accuracy of proxy‐reported height, weight and derived BMI among adults with intellectual disability. Methods Proxies were asked to report height and weight on behalf of adults with intellectual disability who participate in Special Olympics Ontario; their answers were compared to measured height and weight. Results Proxies reported height and weight accurately; the sensitivity of proxy reports for classifying individuals with intellectual disability as overweight and/or obese was 84.6%. Conclusion Proxy reports may be useful when direct measurements of individuals with intellectual disability are not available.