Recent studies suggest a possible comorbidity between Attention-Deficit/Hyperactivity Disorder (ADHD) and obesity. To gain insight into this potential association, we performed a systematic review of the literature excluding case reports, non-empirical studies, and studies not using ADHD diagnostic criteria. Empirically based evidence suggests that obese patients referred to obesity clinics may present with higher than expected prevalence of ADHD. Moreover, all reviewed studies indicate that subjects with ADHD are heavier than expected. However, data on the prevalence of obesity in subjects with ADHD are still limited. As for the mechanisms underlying the potential association between ADHD and obesity, ADHD might lead to obesity via abnormal eating behaviors, impulsivity associated with binge eating might contribute to ADHD in obese patients, or, alternatively, both obesity and ADHD might be the expression of common underlying neurobiological dysfunctions, at least in a subset of subjects. In patients with obesity and ADHD, both conditions might benefit from common therapeutic strategies. Further empirically based studies are needed to understand the potential comorbidity between obesity and ADHD, as well as the possible mechanisms underlying this association. This might allow a more appropriate clinical management and, ultimately, a better quality of life for patients with both obesity and ADHD.