Anger regulation is a challenge for children with autism spectrum disorders (ASD). We investigated if attention-based cognitive behavioral treatment, based on mindfulness cognitive therapy (MBCT) and dialectical behavior therapy (DBT), reduces aggressive behavior and improves anger coping in school-aged autistic children (n = 51). Children were randomized to an active-control or a treatment condition. The treatment included nine weekly sessions attention-based individual therapy. Parents in both conditions received three weekly psychoeducation group sessions to heighten awareness of expressed emotion (EE). For aggressive behavior, treatment reduced temper tantrums and arguing. No effect was found on destroying things and physical violence. For anger coping, treatment increased adaptive coping strategies of diffusion and social support seeking, but had no effect on assertion, rumination, and maladaptive coping direct anger out and avoidance. Treatment did not impact secondary outcome measures concerning children's quality of life (QoL) and parental stress-levels and psychological well-being. In conclusion, school-aged autistic children are able to acquire self-regulation skills reducing temper tantrums and arguing and increasing the use of adaptive anger coping strategies. The intervention shows potential to improve behavior and regulation, but little transfer to other domains. Limitations and future directions involving the child's social environment, including parents, siblings, and teachers are discussed. LAY SUMMARY: Children on the autism spectrum often show aggressive behavior. Treatment can train children to be more aware of their emotions. This study found that this can help reducing temper tantrums and arguing and increasing some coping skills, though no impact was found on several other domains of aggression and coping.