BACKGROUND: Major depression represents a pressing challenge for health care. In England, Increasing Access to Psychological Therapies (IAPT) services provide evidence-based psychological therapies in a stepped-care approach to patients with depression. While introduction of these services has successfully increased access to therapy, estimates suggest that about 50% of depressed patients who have come to the end of the IAPT pathway still show significant levels of symptoms. This study will investigate whether Mindfulness-Based Cognitive Therapy (MBCT), a group intervention combining training in mindfulness meditation and elements from cognitive therapy, can have beneficial effects in depressed patients who have not responded to high-intensity therapy in IAPT. It will seek to establish the effectiveness and cost-effectiveness of MBCT as compared to the treatment these patients would usually receive. METHODS: In a 2-arm randomised controlled trial, patients who currently meet the criteria for major depressive disorder and who have not sufficiently responded to at least 12 sessions of IAPT high-intensity therapy will be allocated, at a ratio of 1:1, to receive either MBCT (in addition to treatment as usual [TAU]) or continue with TAU only. Assessments will take place at baseline, 10 weeks and 34 weeks post-randomisation. The primary outcome will be reduction in depression symptomatology 34 weeks post-randomisation as assessed using the Public Health Questionnaire-9 (PHQ-9). Secondary outcomes will include depressive symptomatology at 10 weeks post-randomisation and other clinical outcomes measured at 10-week and 34-week follow-up, along with a series of binarised outcomes to indicate clinically significant and reliable change. Evaluations of cost-effectiveness will be based on assessments of service use costs collected using the Adult Service Use Schedule and health utilities derived from the EQ-5D. DISCUSSION: This trial will add to the evidence base for the use of MBCT in depressed treatment non-responders. It will constitute the first trial to test MBCT following non-response to psychological therapy, with results providing a direct estimate of efficacy within the IAPT pathway. As such, its results will offer an important basis for decisions regarding the adoption of MBCT for non-responders within IAPT. TRIAL REGISTRATION: ClinicalTrials.gov NCT05236959. Registered on 11 February 2022. ISRCTN 17755571. Registered on 2 February 2021.