Background: Adjunctive psychological interventions improve outcomes in bipolar disorder (BD), but people in latter, more refractory illness stages likely have different clinical needs. The objective here was to test the hypothesis that for people with >‾ 10 episodes of BD, an online mindfulness-based intervention improves quality of life (QoL) relative to a quality-matched psychoeducation control. Methods: A rater-blind, pragmatic superiority randomised clinical trial compared Mindfulness with active control (Psychoeducation). Both interventions were 5-week coach-supported self-management programs with treatment as usual continued. Key inclusion criteria were age 18 to 65 years, confirmed diagnosis of BD, and history of >‾ 10 episodes. Measures were collected at baseline, immediate post-intervention, and 3- and 6- month follow-ups. Main outcome was QoL, measured on the self-report Brief Quality of Life in Bipolar Disorder (Brief QoL.BD) at 5 weeks, using intention-to-treat analyses. The study setting was online, and randomisation was automated into the platform. The trial was prospectively registered on 23 June 2017 through ClinicalTrials.gov (http://www.clinicaltrials.gov, NCT03197974). Findings: Recruitment commenced 19 th September 2017, randomisation target reached 28 th November 2018 (N = 302 randomised) and follow-up data collection completed 6 th July 2019. The primary hypothesis of significant improvement in QoL with Mindfulness relative to Psychoeducation was not supported (Treatment X Time β = -0.69, 95% CI: -2·69, 1·31, p = 0·50). Recruitment was feasible, the platform was safe, both interventions were highly acceptable, but usage was suboptimal. Post hoc analyses found both interventions effective for participants not in remission from depression at baseline. Interpretation: In people with latter-stage BD, an online Mindfulness intervention was not superior to Psychoeducation. More effective future variants could combine mindfulness with psychoeducation content, improve engagement via personalised tailoring of QoL goals, and target more symptomatic individuals. Trial Registration: The trial was prospectively registered on 23 June 2017 through ClinicalTrials.gov (http://www.clinicaltrials.gov, NCT03197974). Funding: Supported by Australian National Health and Medical Research Council (NHMRC; APP1102097). Declaration of Interest: GM has received Grant/Research support from National Health and Medical Research Council (NHMRC), BeyondBlue Barbara Dicker Foundation, Readiness P/L, SiSU Wellness P/L – unrelated to this work, with the exception of the NHMRC funding for the project described here. MB has received Grant/Research Support from the NIH, Cooperative Research Centre, Simons Autism Foundation, Cancer Council of Victoria, Stanley Medical Research Foundation, Medical Benefits Fund, National Health and Medical Research Council, Medical Research Futures Fund, Beyond Blue, Rotary Health, A2 milk company, Meat and Livestock Board, Woolworths, Avant and the Harry Windsor Foundation, has been a speaker for Abbot, Astra Zeneca, Janssen and Janssen, Lundbeck and Merck and served as a consultant to Allergan, Astra Zeneca, Bioadvantex, Bionomics, Collaborative Medicinal Development, Janssen and Janssen, Lundbeck Merck, Pfizer and Servier – all unrelated to this work. NT has received Grant/Research Support from the Australian Government Department of Health, Australian Government Department of Veterans Affairs, Barbara Dicker Brain Sciences Foundation, NHMRC, Mental Health Commission, VicHealth and Victorian State Government Department of Health and Human Services, all unrelated to this work. LB has received Grant/Research Support from the NIH, Australasian Society for Bipolar & Depressive Disorders Ltd, GMHBA, National Health and Medical Research Council, and Servier – all unrelated to this work. SJ has received Grant/Research support from NIHR HTA, EME, RfPB and PGfAR streams all unrelated to this work. Ethical Approval: Swinburne University of Technology Human Research Ethics Committee approved the online trial (2016/289) with international recruitment.