Objective: The aim of this study was to determine whether the serotonin transporter-linked polymorphic region (5-HTTLPR) L (long)/S (short) polymorphism is associated with susceptibility to ADHD. Method: We conducted a meta-analysis of case-control associations and the transmission disequilibrium test (TDT) between the 5-HTTLPR L/S polymorphism and ADHD. Results: A total of 19 comparison studies were included in this meta-analysis. Our meta-analysis of the case-control studies showed no association between ADHD and the 5-HTTLPR S allele, for all study participants (odds ratio [OR] = 1.075, 95% confidence interval [CI] = [0.990, 1.167], p = .085), or for the European or Asian population. The TDT indicated no association between ADHD and the 5-HTTLPR S allele, for all study participants (OR = 1.078, 95% CI = [0.962, 1.207], p = .196). Conclusion: This meta-analysis of case-control studies and TDT showed a lack of association between the 5-HTTLPR L/S polymorphism and ADHD.