INTRODUCTION: Bipolar disorder affects around 2% of the population and is linked with reduced life expectancy and socioeconomic burden. Depressive episodes are difficult to treat and typically more prevalent, enduring and burdensome than manic episodes. The use of antidepressants alone has limited effect and is associated with significant clinical risk through polarity switch. Current National Institute for Health and Care Excellence guidelines recommend quetiapine, olanzapine (with or without fluoxetine) and lamotrigine; however, these medications have limited efficacy, tolerability and acceptability. The ASCEnD study aims to assess the clinical and cost-effectiveness of aripiprazole plus sertraline compared with quetiapine, offering potential improvements for outcomes in bipolar depression. The study is funded by the National Institute for Health and Care Research Health Technology Assessment programme (NIHR132773). METHODS AND ANALYSIS: ASCEnD is a prospective, two-arm, superiority, individually 1:1 randomised, controlled, pragmatic, parallel group, type A open-label clinical trial of aripiprazole/sertraline medication combination compared with quetiapine for bipolar depression. The study is conducted in the UK National Health Service setting with the aim of recruiting and randomising 270 participants followed-up for 24 weeks. Adults with bipolar disorder self-refer or are recruited through primary and secondary care services. The primary outcome is change in depressive symptoms 12-16 weeks after randomisation. Secondary outcomes include measures of symptom change, treatment satisfaction, tolerability, medication adherence, concomitant medication use, psychosocial functioning, quality of life and cost-effectiveness and informal carer measures of quality of life and costs of caring. The exploratory outcome is change in participant reward and punishment responsiveness. Analysis will follow a prespecified statistical analysis plan. A nested qualitative study is included to examine feasibility and acceptability of the trial design. ETHICS AND DISSEMINATION: A Clinical Trial Authorisation from Medicines and Healthcare products Regulatory Agency, and approval from the Health Research Authority (IRAS 1007468) and North East - Newcastle and North Tyneside 1 Research Ethics Committee (23/NE/0132) were obtained. Results will be disseminated through peer-reviewed publications, conference presentations and lay summaries for participants and patient and public groups. TRIAL REGISTRATION NUMBER: ISRCTN63917405.