There is great variability in the degree to which children with attention deficit/hyperactivity disorder (ADHD) improve through behavioral treatments. This study investigates the influence of the dopamine transporter gene (SCL6A3/DAT1) on outcome of behavioral parent training (BPT). Study subjects were a subsample (n = 50, for whom DAT1 genotypes were available) of a randomized controlled BPT effectiveness study (N = 94) comparing BPT plus ongoing routine clinical care (RCC) versus RCC alone in referred children (4–12 years old) with ADHD. Treatment outcome was based on parent-reported ADHD symptoms and behavioral problems. Presence of 2 versus no or 1 DAT1 10-repeat allele served as moderator variable. Time × Treatment × Genotype effect was analyzed with repeated-measures analysis of variance, controlling for baseline medication status. Results indicate that DAT1 moderated treatment response (p = .009). In children with no or 1 DAT1 10-repeat allele, superior treatment effects of BPT + RCC compared with RCC alone were present (p = .005), which was not the case in children with 2 DAT1 10-repeat alleles (p = .57). Our findings suggest that genetic differences in DAT1 in children with ADHD influence their susceptibility to a behavioral intervention directed at shaping their environment through their parents. The role of the dopamine system in motivation and learning and in the aberrant sensitivity to reinforcement in children with ADHD may explain this moderating effect, given that the management of contingencies is typically addressed in BPT.