BACKGROUND: Mindfulness-based cognitive therapy (MBCT) seems a promising intervention for bipolar disorder (BD), but there is a lack of randomised controlled trials (RCT) investigating this. The purpose of this multicentre, evaluator blinded RCT was to investigate the added value of MBCT to treatment as usual (TAU) in BD up to 15 months follow-up (NCT03507647). METHODS: A total of 144 participants with BD type I and II were randomised to MBCT + TAU (n = 72) and TAU (n = 72). Primary outcome was current depressive symptoms. Secondary outcomes were current (hypo)manic and anxiety symptoms, recurrence rates, rumination, dampening of positive affect, functional impairment, mindfulness skills, self-compassion, and positive mental health. Potential moderators of treatment outcome were examined. RESULTS: MBCT + TAU was not more efficacious than TAU in reducing current depressive symptoms at post-treatment (95% CI [-7.0 to 1.8], p = 0.303, d = 0.24) or follow-up (95% CI [-2.2 to 6.3], p = 0.037, d = 0.13). At post-treatment, MBCT + TAU was more effective than TAU in improving mindfulness skills. At follow-up, TAU was more effective than MBCT + TAU in reducing trait anxiety and improving mindfulness skills and positive mental health. Exploratory analysis revealed that participants with higher depressive symptoms and functional impairment at baseline benefitted more from MBCT + TAU than TAU. CONCLUSIONS: In these participants with highly recurrent BD, MBCT may be a treatment option in addition to TAU for those who suffer from moderate to severe levels of depression and functional impairment. TRIAL REGISTRATION: ClinicalTrials.gov, NCT03507647. Registered the 25 April 2018, https://www.clinicaltrials.gov/ct2/show/NCT01126827.