Background: Brief Contact Interventions (BCIs) have been of increasing interest to suicide prevention clinicians, researchers and policy makers. However, there has been no systematic assessment into the mechanisms underpinning BCIs. The aim of the current paper is to provide a systematic review of the proposed mechanisms underpinning BCIs across trial studies. Method: A systematic review was conducted of trials using BCIs (post-discharge telephone contacts; emergency or crisis cards; and postcard or letter contacts) for suicide or self-harm. Following PRISMA guidelines, we searched CENTRAL, MEDLINE, EMBASE, and the reference lists of all past reviews in the area. Secondary searches of reference lists were undertaken. Results: Sixteen papers provided a description of possible mechanisms which we grouped into three main areas: social support; suicide prevention literacy, and; learning alternative coping behaviours. After assessment of the studies and considering the plausibility of mechanisms, we suggest social support and improved suicide prevention literacy are the most likely mechanisms underpinning BCIs. Conclusion: Researchers need to better articulate and measure the mechanisms they believe underpin BCIs in trial studies. Understanding more about the mechanisms of BCIs' will inform the development of future interventions for self-harm and suicide.