Objective: Fetal alcohol spectrum disorders (FASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are associated with a range of neurocognitive impairments. Executive functioning deficits are a hallmark feature of both disorders. Method: The present meta-analysis was undertaken to disentangle the behavioral phenotype of FASD and ADHD by quantitatively differentiating executive functioning differences between these two groups. The current meta-analysis reviews 15 studies comparing children and adolescents with FASD and ADHD to typically developing (TD) samples, on a variety of executive function measures. Results: Results indicate that when compared with TD samples, FASD and ADHD samples demonstrate significant executive function deficits (d = 0.82 and d = 0.55, respectively). In addition, FASD samples experience significantly greater deficits when compared with ADHD samples (d = 0.25). Results were moderated by IQ and socioeconomic status. Conclusion: These findings further our understanding of the cognitive differences between FASD and ADHD samples and have the potential to influence future basic research, assessment, and intervention.