Working memory deficits are consistently found in ADHD children, which might underlie core ADHD symptoms, hindering ADHD children’s academic achievement. Thus, one way to enhance ADHD children’s academic achievement is to mitigate their working memory deficits through working memory training. A widely applied training is Cogmed Working Memory Training (WMT). Despite the prevalence of this training, its effect on ADHD children has been rarely reviewed. This study aims to fill this gap by systematically reviewing the effect of Cogmed WMT on ADHD school-age children’s working memory, ADHD symptoms, and academic achievement. It systematically searched PsycINFO, Google Scholar (for accessing grey literature), and Cogmed websites. Eleven randomised controlled trials met the eligibility criteria. Findings of these studies were qualitatively synthesised. The internal and external validity of studies included in this review were critically assessed. Results showed that Cogmed WMT might have a positive effect on school-age ADHD children’s performance on trained working memory tasks. However, the effect of this training was spurious for untrained working memory tasks, ADHD symptoms, and academic achievement. Findings of this study therefore did not yield strong support for Cogmed WMT having a positive effect on ADHD school-age children’s academic achievement. Hence, educational practitioners need to maintain a critical attitude when considering whether to adopt Cogmed WMT for ADHD children. More research on the effect of Cogmed WMT on ADHD children is also needed.