Objective: To compare a novel "third wave" mindfulness-based training program with an established skills training derived from dialectical behavior therapy, to reduce ADHD symptoms and improve mindfulness and self-efficacy. Method: Ninety-one adults with ADHD (combined and inattentive type, mainly medicated) were non-randomly assigned to and treated within a mindfulness-based training group (MBTG, n = 39) or a skills training group (STG, n = 52), each performed in 13 weekly 2-hr sessions. Results: General linear models with repeated measures revealed that both programs resulted in a similar reduction of ADHD symptoms, and improvement of mindfulness and self-efficacy. However, the effect sizes were in the small-to-medium range. A decrease in ADHD symptoms ≥30% was observed in 30.8% of the MBTG participants and 11.5% of the STG participants. Conclusion: The comparatively weak results may be due to limitations such as the absence of randomization, the lack of a control group without intervention, and the lack of matching groups for borderline, depression, and anxiety status. Moreover, audio instructions for home exercises and more stringent monitoring of participants’ progress and eventual absence from sessions might have improved the outcome.