Background: According to the World Health Organization, 727,000 people die by suicide worldwide, corresponding to one death every 43 seconds [1]. For every suicide, an estimated 135 individuals are exposed and around 60 are significantly affected [2]. People bereaved by suicide experience elevated levels of shame, guilt, and social isolation, which can complicate adaptation to the loss. Prolonged grief disorder is more prevalent following violent losses and is associated with psychiatric comorbidity, reduced quality of life, and increased suicide risk [3]. Despite this heightened vulnerability, the effectiveness of mental health interventions for suicide bereavement remains unclear [4]. Aim: To evaluate the benefits and harms of mental health interventions for adults bereaved by suicide and to summarise the characteristics of interventions tested in randomised controlled trials (RCTs). Methods: This systematic review followed Cochrane Handbook guidance and PRISMA 2020. We included RCTs evaluating any mental health intervention for adults bereaved by suicide. Multiple databases (Web of Science, Embase, PubMed, CENTRAL, PsycINFO, CINAHL, BVS/LILACS) were searched from inception to October 01, 2025. Two reviewers independently screened studies, extracted data, assessed risk of bias using the Cochrane RoB 2 tool and judged certainty of the evidence with GRADE framework. Primary outcomes were severity of complicated grief symptoms (The Inventory of Complicated Grief) and suicidal ideation (e.g., Beck Scale for Suicidal Ideation), with post-treatment as the main timepoint. Where appropriate, random-effects meta-analyses were conducted. The review followed a preregistered protocol (CRD420251135641). Results: To date, six RCTs were included, all at high risk of bias, and the certainty of the evidence was very low for all comparisons. A meta-analysis of two trials of CBT-based interventions delivered online demonstrated a significant reduction in grief severity compared with waitlist controls (MD = -3.56; 95% CI = -6.77 to -0.35; very low certainty). Two other trials of counselling and psychoeducational interventions showed no significant effects compared with care as usual. No significant effects were observed for suicidal ideation across two trials. Authors of bereavement RCTs were contacted to provide individual participant data for suicide-bereaved subsamples; data from eligible trials are currently being prepared and will be incorporated in the final publication. Conclusion: Effects of mental health interventions for suicide bereavement were inconsistent and supported by very low certainty evidence. To build a united future for suicide prevention, people bereaved by suicide, who face elevated suicide risk themselves, must be a central focus of rigorous trials and scalable postvention interventions.