Autism spectrum disorder and attention deficit/hyperactivity disorder are neurodevelopmental disorders with high rates of co-occurrence. However, there is a dearth of large, nationally representative studies examining the prevalence of co-occurring autism spectrum disorder and attention deficit/hyperactivity disorder among children. The aim of this study was to estimate the prevalence of parent-reported co-occurring autism spectrum disorder and attention deficit/hyperactivity disorder in the United States and examine correlates of co-occurring autism spectrum disorder and attention deficit/hyperactivity disorder by sociodemographic and household factors. Data were analyzed from the 2016-2018 National Survey of Children's Health. A total of 88,051 children aged 3-17 years were included in the analysis. Bivariate and multivariable analyses were conducted to assess the associations between sociodemographic and household characteristics and current co-occurrence of autism spectrum disorder and attention deficit/hyperactivity disorder. An estimated 1.2% of children (740,816) aged 3-17 years had co-occurring autism spectrum disorder and attention deficit/hyperactivity disorder. Sex, age, race/ethnicity, health status, and health insurance were associated with having co-occurring autism spectrum disorder and attention deficit/hyperactivity disorder. Children who were Black, non-Hispanic (adjusted odds ratio = 0.63; 95% confidence interval [0.40, 0.99]) or multi-racial/other, non-Hispanic (adjusted odds ratio = 0.58; 95% confidence interval [0.42, 0.80]) had significantly lower odds of co-occurring autism spectrum disorder and attention deficit/hyperactivity disorder than White non-Hispanics. Findings suggest implementing early developmental screening and surveillance for co-occurring autism spectrum disorder and attention deficit/hyperactivity disorder and coordinating strategies that optimize early identification and intervention for all children suspected of having co-occurring autism spectrum disorder and attention deficit/hyperactivity disorder, particularly those from underrepresented groups.