ADHD affects working memory (WM) and other executive functions (EFs) and thereby negatively impacts school performance, clinical symptoms and functional impairment. The main aim of this study was to analyse the efficacy of computerized WM training (CWMT) on EF rating scales. A secondary objective was to assess its efficacy on performance-based measures of EF (PBMEF), learning, clinical symptoms and functional impairment. 66 children with combined-type ADHD between 7 and 12 years of age from the Child and Adolescent Psychiatric Unit (Spain) were included in this randomized, double-blind, placebo-controlled, parallel-group clinical trial. The participants were randomized (1:1) to an experimental group (EG) (CWMT) (n = 36) or a control group (CG) (placebo training). Assessments were conducted at baseline (T0), 1-2 weeks (T1), and 6 months post-intervention (T2) with the administration of EF rating scales, PBMEF, measures of academic achievement, and questionnaires regarding clinical symptoms and functional impairment. Participants, parents, teachers and professionals who performed the cognitive assessments were blinded. Adjusted multiple linear regression analysis showed significant improvements in EF scales-parent version, from T1 to T2, on the metacognition index [p = 0.03, d’ = -0.78 (95 % CI -1.28 to -0.27)] and on WM (also significant at T2-T0) and plan/organize subscales. Significant improvements were also noted in EF scales-teacher version, from T0 to T1 and T2, on the metacognitive index [p = 0.05, d’ = -0.37 (95 % CI -0.86 to 0.12) T1-T0, p = 0.02, d’ = -0.81 (95 % CI -1.31 to -0.30) T2-T0] and on the initiate, WM, monitor and shift subscales. There were also significant improvements in PBMEF, ADHD symptoms, and functional impairment. CWMT had a significant impact on ADHD deficits by achieving long-term far-transfer effects.