Objective: Bipolar depression is characterized by neurobiological features including perturbed oxidative biology, reduction in antioxidant levels, and a concomitant rise in oxidative stress markers. Bipolar depression manifests systemic inflammation, mitochondrial dysfunction, and changes in brain growth factors. The depressive phase of the disorder is the most common and responds the least to conventional treatments. Garcinia mangostana Linn, commonly known as mangosteen, is a tropical fruit. The pericarp’s properties may reduce oxidative stress and inflammation and improve neurogenesis, making mangosteen pericarp a promising add-on therapy for bipolar depression. Methods: Participants will receive 24 weeks of either 1,000 mg mangosteen pericarp or placebo per day, in addition to their usual treatment. The primary outcome is change in severity of mood symptoms, measured using the Montgomery-Åsberg Depression Rating Scale (MADRS), over the treatment phase. Secondary outcomes include global psychopathology, quality of life, functioning, substance use, cognition, safety, biological data, and cost-effectiveness. A follow-up interview will be conducted 4 weeks post-treatment. Conclusion: The findings of this study may have implications for improving treatment outcomes for those with bipolar disorder and may contribute to our understanding of the pathophysiology of bipolar depression. Clinical trial registration: Australian and New Zealand Clinical Trial Registry, ACTRN12616000028404.