Introduction The current randomised controlled trial will evaluate the efficacy of an internet-delivered positive imagery cognitive bias modification (CBM) intervention for depression when compared with an active control condition and help establish the additive benefit of positive imagery CBM when delivered in combination with internet cognitive behavioural therapy for depression.

Methods and analysis Patients meeting diagnostic criteria for a current major depressive episode will be recruited through the research arm of a not-for-profit clinical and research unit in Australia. The minimum sample size for each group (α set at 0.05, power at 0.80) was identified as 29, but at least 10% more will be recruited to hedge against expected attrition. We will measure the impact of CBM on primary measures of depressive symptoms (Beck Depression Inventory—second edition (BDI-II), Patient Health Questionnaire (PHQ9)) and interpretive bias (ambiguous scenarios test-depression), and on a secondary measure of psychological distress (Kessler-10 (K10)) following the 1-week CBM intervention. Secondary outcome measures of psychological distress (K10), as well as disability (WHO disability assessment schedule-II), repetitive negative thinking (repetitive thinking questionnaire), and anxiety (state trait anxiety inventory-trait version) will be evaluated following completion of the 11-week combined intervention, in addition to the BDI-II and PHQ9. Intent-to-treat marginal and mixed effect models using restricted maximum likelihood estimation will be used to evaluate the primary hypotheses. Clinically significant change will be defined as high-end state functioning (a BDI-II score <14) combined with a total score reduction greater than the reliable change index score. Maintenance of gains will be assessed at 3-month follow-up.

Ethics and dissemination The current trial protocol has been approved by the Human Research Ethics Committee of St Vincent's Hospital and the University of New South Wales, Sydney.

Trial registration Australian New Zealand Clinical Trials Registry: ACTRN12613000139774 and Clinicaltrials.gov: NCT01787513. This trial protocol is written in compliance with the Standard Protocol Items: recommendations for Interventional Trials (SPIRIT) guidelines.