Objectives ADHD has a considerable economic impact. The aim of this study is to conduct a trial-based economic evaluation of mindfulness-based cognitive therapy (MBCT) added to treatment as usual (TAU) versus TAU in adults with ADHD. Methods A Dutch economic evaluation with a time horizon of 9 months was conducted from the societal perspective in the intention-to-treat (ITT) sample. Costs were assessed with a self-report questionnaire. Outcomes were expressed in quality adjusted life years (QALYs) and response rate. Bootstrap simulations were performed to estimate mean costs, QALYs, response rate, incremental cost-effectiveness ratios (ICERs), and associated uncertainty. Additional sensitivity analyses were done with imputed data, without extreme cost outliers, using the per protocol sample, and from a health care perspective. Results In the ITT sample, societal costs were €3572 for MBCT + TAU (n = 47; 95% CI 2416 to 4995) and €3389 for TAU (n = 49; 2327 to 4763). Average QALYs were 0.542 (0.522 to 0.563) per patient for MBCT + TAU and 0.534 (0.511 to 0.556) for TAU. In MBCT + TAU, more patients responded than in TAU (31% versus 6%; M bootstrapped difference 25%, 12 to 40%). ICERs were €21,963 per QALY gained and €389 per responder. At a threshold of €30,000 per QALY, the probability of MBCT being cost-effective was 51%. All sensitivity analyses showed more favorable results for MBCT + TAU. Conclusions In most analyses, MBCT was found to be more costly and effective, particularly in terms of disease-specific outcome, than TAU. If the threshold exceeds €30,000 per QALY and €1000 per responder, MBCT seemed cost-effective in treating adult ADHD. Keywords Adult ADHD. Mindfulness-based cognitive therapy. Cost-effectiveness. Health economy. Quality of life Attention-deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder with an estimated prevalence of 2.5% at adult age (Simon et al. 2009). Its core symptoms affect academic, professional, and social functioning (Karlsdotter et al. 2016), and have a considerable personal and economic impact. A systematic review of Doshi et al. (2012) showed that in the USA, the estimates of overall annual incremental (excess)