Attention-deficit/hyperactivity disorder (ADHD) is a common heritable childhood-onset psychiatric disorder with impaired visual memory. Based on the evidence from treatment effect of atomoxetine, which interacts directly with the norepinephrine transporter, on visual memory in children with ADHD, this study examined the linkage disequilibrium structure of the norepinephrine transporter gene (SLC6A2) and the association between SLC6A2 and ADHD and visual memory, a promising endophenotype for ADHD. This family-based association sample consisted of 382 probands with DSM-IV ADHD and their family members (n = 1298 in total) of Han Chinese in Taiwan. Visual memory was assessed by the Pattern Recognition Memory (PRM) and Spatial Recognition Memory (SRM) tasks of the Cambridge Neuropsychological Test Automated Battery (CANTAB). We screened 21 polymorphisms across SLC6A2 and used the Family-Based Association Test (FBAT) to test the associations of SLC6A2 polymorphisms with ADHD and the PRM and SRM measures. In haplotype analyses, a haplotype rs36011 (T)/rs1566652 (G) was significantly associated with ADHD (minimal p = 0.045) after adjustment for multiple testing. In quantitative analyses, this TG haplotype also demonstrated significant associations with visual memory measures, including mean latency of correct responses in PRM (minimal p = 0.019), total correct responses in PRM (minimal p = 0.018), and total correct responses in SRM (minimal p = 0.015). Our novel finding of the haplotype rs36011 (T)/rs1566652 (G) as a novel genetic marker involved in both ADHD disease susceptibility and visual memory suggests that allelic variations in SLC6A2 could provide insight into the pathways leading from genotype to phenotype of ADHD.