Objective: The aim of this study was to examine the effectiveness of mindfulness as a treatment for adults diagnosed with ADHD. A 12-week-adapted mindfulness-based cognitive therapy (MBCT) program is compared with a waiting list (WL) group. Method: Adults with ADHD were randomly allocated to MBCT (n = 55) or waitlist (n = 48). Outcome measures included investigator-rated ADHD symptoms (primary), self-reported ADHD symptoms, executive functioning, depressive and anxiety symptoms, patient functioning, and mindfulness skills. Results: MBCT resulted in a significant reduction of ADHD symptoms, both investigator-rated and self-reported, based on per-protocol and intention-to-treat analyses. Significant improvements in executive functioning and mindfulness skills were found. Additional analyses suggested that the efficacy of MBCT in reducing ADHD symptoms and improving executive functioning is partially mediated by an increase in the mindfulness skill "Act With Awareness." No improvements were observed for depressive and anxiety symptoms, and patient functioning. Conclusion: This study provides preliminary support for the effectiveness of MBCT for adults with ADHD.