Objective The aim of this study was to clarify the association between organophosphate pesticides (OPs) and attention-deficit/hyperactivity disorder (ADHD) related to oxidative stress and genetic polymorphisms. Methods This case-control study enrolled 93 children with ADHD and 112 control children in north Taiwan. Six dialkyl phosphate (DAP) metabolites of OPs and oxidative stress biomarkers were analyzed. Polymorphisms of the dopamine receptor D4 gene (DRD4) were identified. Results Children with ADHD had significantly higher dimethylphosphate (DMP, 236.69 nmol/g cre. vs. 186.84 nmol/g cre., p value = 0.01) and 4-hydroxy-2-nonenal-mercapturic acid (HNE-MA, 28.95 µg/g cre. vs. 16.55 µg/g cre., p value<0.01) concentrations than control children. Children who carried DRD4 GA/AA genotypes (rs752306) were less likely than those who carried the DRD4 GG genotype to have ADHD (odds ratio [OR]: 0.45, 95% CI: 0.24–0.84). The estimated value of the AP (attributable proportion due to interaction) was 0.59 (95% CI: 0.13–1.05), indicating that 59% of ADHD cases in DMP-exposed children with the DRD4 GG genotype were due to the gene-environment interaction. After adjustment for other covariates, children who carried the DRD4 GG genotype, had been exposed to high DMP levels (more than the median), and had high HNE-MA levels had a significantly increased risk for developing ADHD (OR = 11.74, 95% CI: 2.12–65.04). Conclusion This study indicated a gene-environment interaction in the risk of ADHD in children. The association between DMP and ADHD in children might relate to the mechanism of lipid peroxidation. Dose-response relationships and the combined effects of OPs, oxidative stress, and genetic polymorphism on ADHD should not be neglected.