BACKGROUND: Depression has serious personal, family and economic consequences. It is estimated that it will cost £12.15 billion to the economy each year in England by 2026. Improving access to psychological therapies (IAPT) is the National Health Service talking therapies service in England for adults experiencing anxiety or depression. Over 1 million people are referred to IAPT every year, over half experiencing depression. Where symptoms of depression are mild to moderate, people are typically offered cognitive behavioural therapy (CBT) self-help (CBT-SH) supported by a psychological well-being practitioner. The problem is that over half of people who complete treatment for depression in IAPT remain depressed despite receiving National Institute of Health and Care Excellent recommended treatment. Furthermore, less than half of IAPT service users complete treatment. This study seeks to investigate the effectiveness of an alternative to CBT-SH. Mindfulness-based cognitive therapy (MBCT) differs from CBT in focus, approach and practice, and may be more effective with a higher number of treatment completions. METHODS/DESIGN: This is a definitive randomised controlled trial comparing supported MBCT self-help (MBCT-SH) with CBT-SH for adults experiencing mild to moderate depression being treated in IAPT services. We will recruit 410 participants experiencing mild to moderate depression from IAPT services and randomise these to receive either an MBCT-based self-help workbook or a CBT-based self-help workbook. Participants will be asked to complete their workbook within 16 weeks, with six support sessions with a psychological well-being practitioner. The primary outcome is depression symptom severity on treatment completion. Secondary outcomes are treatment completion rates and measures of generalized anxiety, well-being, functioning and mindfulness. An exploratory non-inferiority analysis will be conducted in the event the primary hypothesis is not supported. A semi-structured interview with participants will guide understanding of change processes. DISCUSSION: If the findings from this randomised controlled trial demonstrate that MBCT-SH is more effective than CBT-SH for adults experiencing depression, this will provide evidence for policy makers and lead to changes to clinical practice in IAPT services, leading to greater choice of self-help treatment options and better outcomes for service users. If the exploratory non-inferiority analysis is conducted and this indicates non-inferiority of MBCT-SH in comparison to CBT-SH this will also be of interest to policy makers when seeking to increase service user choice of self-help treatment options for depression. TRIAL REGISTRATION: Current Controlled Trial registration number: ISRCTN 13495752. Registered on 31 August 2017 (www.isrctn.com/ISRCTN13495752).