Summary Individuals with autism spectrum disorder (ASD) or attention‐deficit hyperactivity disorder (ADHD) may have unhealthy bodyweight. This meta‐analysis was performed to understand the weight status in individuals with ASD or ADHD. PubMed, Embase, Cochrane and ISI Web of Science databases were searched from inception until June 2020 to identify relevant studies. Prevalence estimates and their 95% confidence intervals (CIs) of obesity, overweight and underweight were separately pooled using random‐effects models. A total of 95 studies were included in the meta‐analysis. The pooled estimates of the prevalence of obesity, overweight and underweight were 21.8%, 19.8% and 6.4% in individuals with ASD and 14.7%, 20.9% and 4.0% in individuals with ADHD. In subgroup analyses, an increasing trend in the prevalence of unhealthy weight was observed from children aged 2 to 5 years to adults with ASD (obesity: from 16.7% to 31.3%, overweight: from 16.2% to 27.2%, underweight from 5.3% to 8.6%) and from children aged 6 to 12 years to adults with ADHD (obesity: from 13.5% to 19.3%, overweight: from 18.8% to 31.2%). The worrisome epidemic of obesity and overweight in individuals with ASD, ADHD highlighted the need for weight management.