PURPOSE: Irritable bowel syndrome (IBS) is a functional gastrointestinal (GI) disorder characterized by abdominal pain and altered bowel habits. It is estimated to affect 10-22% of the UK population. The use of psychological interventions in IBS is increasingly empirically supported, but little is known about the mechanism of psychological treatment approaches. The present systematic review aimed to investigate the mechanisms of psychological treatment approaches applied to IBS. METHODS: The systematic review included studies conducting mediation analysis in the context of psychological interventions for IBS, focusing on the outcomes of symptom severity and/or quality of life (QoL). RESULTS: Nine studies in total were included in the review. Eight of the studies assessed mediation in the context of cognitive behavioural-based interventions, and one study assessed mediation in a mindfulness-based stress reduction intervention. Results indicate that change in illness-specific cognitions is a key process by which psychological treatments may have an effect on the outcomes of symptom severity and QoL. Furthermore, results suggest that whilst GI-specific anxiety may also be a key mechanism of treatment effect, it would appear that general or state anxiety is not. Although less commonly included in mediation analysis, illness-specific behaviours may also have a mediating role. CONCLUSIONS: A mediational model amalgamating the results of studies is proposed to illustrate the findings of the review. The model depicts the process by which psychotherapy changes illness-specific cognitions, behaviours, and anxiety to achieve reduction in symptom severity. Statement of contribution What is already known on this subject? Cognitive behavioural therapy (CBT) is the predominant psychological treatment for irritable bowel syndrome (IBS), although there is some research supporting other treatments such as mindfulness and hypnotherapy. Mediation analysis in the context of psychological treatments for IBS has just begun to explore possible mechanisms of treatment effect especially within CBT studies. Some studies include anxiety in a basic mediation analysis, whilst others include cognitions with inconsistent results for each. What does this study add? Reviews mediators included in mediation analysis and the methods used for mediation analysis Proposes a mediation model informed by the results of the review for future studies to investigate Provides clinical implications for the targeting of cognitions and behaviours rather than general anxiety.