This single-arm, open-label study aimed to investigate the efficacy of a cognitive-behavioural group training based on acceptance and commitment therapy (ACT) on cognition in drug-naïve children with attention deficit hyperactivity disorder (ADHD). Thirty-six children with ADHD aged 8–13 were invited to participate in the 9-month ACT training programme, which consisted of 26 weekly sessions of group therapy lasting 90 min each. Their parents also received 12 sessions of ACT-based parent training, every 2 weeks. The outcome measure for the present study was the change in the cognitive performance assessed by a battery of computerised task. The cognitive outcome of children receiving ACT-group intervention was compared to that of an external untreated control group of children with ADHD. No significant improvements were observed in any of the cognitive measures. This preliminary study suggests that the 9-month ACT-group training programme might not have positive effects on cognitive difficulties usually occurring in ADHD. Future randomised controlled trials with larger sample sizes are required to shed more light on this issue.