Objective The aim of the present study is evaluate the effectiveness of an Acceptance and Commitment Therapy (ACT)- based training protocol, in adjunct to token economy and previous parent training, in a sample of children with AttentionDeficit/Hyperactivity Disorder (ADHD). By promoting the reduction of immediate responses to thoughts and feelings, we aimed to reduce the impulsive behaviour of children and to improve their self-regulation. Methods The protocol was centred on awareness of the present moment, defusion and acceptance of feelings and emotions. Behavioural (Conners’ Parent Rating Scale -Revised: Long version, CPRS-R:L) and severity measures (Clinical Global Impression -Severity, CGI-S) were assessed before and after treatment in a clinical sample of 31 children aged 8–13 years. Results At the end of the ACT protocol, children showed significant improvement in global functioning and behavioural symptoms. There were significant improvements in the CPRS subscales Cognitive Problems (p = 0.005), Hyperactivity (p = 0.006), Perfectionism (p = 0.017), ADHD Index (p = 0.023), Global Index: Restless–Impulsive (p = 0.023), Global Index: Total (p = 0.036), DSM IV Inattentive (p = 0.029), DSM IV Hyperactive–Impulsive (p = 0.016), and DSM IV Total (p = 0.003). When controlling for the confounding effect of pharmacological therapy, comorbidities and socio-economic status, treatment maintained a significant effect on the CPRS subscales Perfectionism (partial η2 = 0.31, p < 0.01), Global Index: Restless–Impulsive (partial η2 = 0.29, p < 0.01), Global Index: Total (partial η2 = 0.31, p < 0.01), DSM IV Hyperactive–Impulsive (partial η2 = 0.20, p = 0.02). Symptom severity as rated by CGI-S scores decreased in 74.2% of the children.