The co-existence of gaming disorder (GD) with other mental health problems has been widely reported. Despite the growing research interest in the comorbidity of GD with attention-deficit/hyperactivity disorder (ADHD), to date, no quantitative synthesis has been performed. The present study comprised a systematic literature search using Scopus, Science Direct, Web of Science, and PubMed databases. Three types of studies were included in the analyses: studies reporting (i) correlation coefficients between the symptoms of GD and ADHD, (ii) means, and standard deviations for comparison of GD severity between ADHD/non-ADHD individuals, and (iii) comparison of ADHD severity between GD/non-GD individuals. The results indicated a moderate relationship between GD and ADHD symptom severity when both subdomains of ADHD were combined (r = 0.296), and also when only inattention (r = 0.306) or hyperactivity (r = 0.266) symptoms were analyzed, which was also confirmed in a structural equation model meta-analysis. Studies showed a large average difference comparing the GD symptom severity of ADHD and non-ADHD individuals (g = 0.693), or ADHD symptom severity of GD and non-GD individuals (g = 0.854). In some cases, higher estimates of association were reported among studies that (i) had a higher proportion of males, (ii) assessed problematic internet use among predominantly videogame player samples rather than assessing only GD, and (iii) had been more recently published. The present review shows that this is an emerging field demonstrating significant results in cross-sectional correlational studies. However, future research should apply more rigorous methodologies to investigate the relationship further (e.g., longitudinal studies and studies using professional/clinical ratings and diagnosis). These results suggest that screening and treatment for ADHD among individuals with gaming disorder is necessary, and individuals with ADHD should be made aware of their higher susceptibility to gaming disorder.