BACKGROUND: There is a high need for evidence-based psychosocial treatments for adult attention-deficit hyperactivity disorder (ADHD) to offer alongside treatment as usual (TAU). Mindfulness-based cognitive therapy (MBCT) is a promising psychosocial treatment. This trial investigated the efficacy of MBCT + TAU v. TAU in reducing core symptoms in adults with ADHD. METHODS: A multicentre, single-blind, randomised controlled trial (ClinicalTrials.gov: NCT02463396). Participants were randomly assigned to MBCT + TAU (n = 60), an 8-weekly group therapy including meditation exercises, psychoeducation and group discussions, or TAU only (n = 60), which reflected usual treatment in the Netherlands and included pharmacotherapy and/or psychoeducation. Primary outcome was ADHD symptoms rated by blinded clinicians. Secondary outcomes included self-reported ADHD symptoms, executive functioning, mindfulness skills, self-compassion, positive mental health and general functioning. Outcomes were assessed at baseline, post-treatment, 3- and 6-month follow-up. Post-treatment effects at group and individual level, and follow-up effects were examined. RESULTS: In MBCT + TAU patients, a significant reduction of clinician-rated ADHD symptoms was found at post-treatment [M difference = -3.44 (-5.75, -1.11), p = 0.004, d = 0.41]. This effect was maintained until 6-month follow-up. More MBCT + TAU (27%) than TAU participants (4%) showed a ⩾30% reduction of ADHD symptoms (p = 0.001). MBCT + TAU patients compared with TAU patients also reported significant improvements in ADHD symptoms, mindfulness skills, self-compassion and positive mental health at post-treatment, which were maintained until 6-month follow-up. Although patients in MBCT + TAU compared with TAU reported no improvement in executive functioning at post-treatment, they did report improvement at 6-month follow-up. CONCLUSIONS: MBCT might be a valuable treatment option alongside TAU for adult ADHD aimed at alleviating symptoms.