Objective: Computerized cognitive training (CCT) as add-on treatment to stimulants for ADHD core symptoms is scarcely investigated. The purpose of this study is to assess the effect of CCT in a randomized controlled clinical trial for ADHD in children and adolescents treated with stimulants. Method: Fifty-three participants aged 6 to 13 years receiving stimulant treatment and presenting ADHD residual symptoms were randomized either to a CCT (n = 29) or to a controlled nonactive condition (n = 24) for four sessions/week during 12 weeks. The main outcome measure was inattentive symptoms assessed using the Swanson, Nolan, and Pelham–IV (SNAP-IV) Scale. Secondary outcomes include, among others, hyperactive/impulsive symptoms and cognitive tests. Results: There were neither significant group differences on ADHD-inattentive symptoms after the intervention nor on both ADHD-hyperactivity/impulsivity symptoms and cognitive measures. Conclusion: Our study does not provide evidence for the benefits of cognitive training over nonactive training on core ADHD symptoms in medicated ADHD children and adolescents.