INTRODUCTION: Major depressive disorder (MDD) significantly impacts millions worldwide, with limited success in achieving remission for many patients, leading to high disease burden and increased suicide risk. Psychotherapy and antidepressants, although effective, do not provide relief for all, prompting the search for alternative treatments. Ketogenic diets have demonstrated positive effects on brain health. Our study aims to investigate the efficacy of the ketogenic diet in alleviating MDD symptoms, filling a critical gap in psychiatric treatment options and offering a novel dietary approach with potential to mitigate disease burden and enhance mental well-being. METHODS: This phase 2 randomized controlled trial will evaluate the efficacy of a 10-week program of dietitian counseling and ketogenic meal provision versus an intervention with similar dietetic contact promoting a healthy, insulin-lowering, non-ketogenic diet. The primary outcome is the change in the Patient Health Questionnaire nine-item depression score. Secondary outcomes include cognitive and affective mindfulness, self-efficacy, sleep, cognitive function, work and social adjustment, and various immunological, metabolic, and microbiome markers at weeks 6 and 10. CONCLUSION: This study addresses a critical gap in depression treatment by exploring the ketogenic diet's potential as a metabolic mood enhancing intervention. Given the global impact of depression and limitations of current therapies, this research is valuable for exploring previously underappreciated neuroprotective and metabolic mechanisms and clinical benefits.