OBJECTIVES: There is evidence to support the effectiveness of tai chi for reducing pain-related symptoms across multiple musculoskeletal conditions, however, the mechanism(s) are unclear. This study explores the role of pain-catastrophising as a possible mediator in the relationship between practising tai chi and back pain symptoms. DESIGN: Exploratory mediation analyses using a Baron and Kenny approach and bootstrapping methods were employed as a secondary analysis of data from a previously published randomised controlled trial. SETTING: Adults with persistent low back pain were recruited via community advertisement in Sydney, NSW, Australia. INTERVENTIONS: A 10-week tai chi intervention and a wait-list control. MAIN OUTCOME MEASURES: Patients completed self-assessments of pain intensity and bothersomeness using 0-10 numerical rating scales, disability using the Roland Morris Disability Questionnaire and pain catastrophising using the Pain Coping Strategies Questionnaire pre and post intervention. RESULTS: The results suggest partial mediation, by which a reduction in catastrophising explained approximately 1/3 of the effect on pain intensity and bothersomeness and 2/3 of the effect on disability. CONCLUSION: As the first known mediation analysis of tai chi for reducing low back pain-symptoms, it provides initial evidence that it may do so by having an effect on cognitive appraisal outcomes such as pain-catastrophising and information for choosing appropriate process measures for future studies. As tai chi is growing in popularity as an alternative treatment for patients with chronic musculoskeletal conditions, it is important to gain a better understanding of how it might work using confirmatory mediation analysis alongside future RCTs.