Purpose: The paper aims to explore the role of union strategic influence on the adoption of High Performance Work Systems (HPWS) in organisations and examines how the effects of job security and then in turn the industrial relations climate, mediate this relationship in a serial manner. Design/methodology/approach: The research analyses an original quantitative survey of union negotiators and representatives in 382 workplaces in England. The analysis employs structural equation modelling techniques to examine the relationships between union influence, job security, industrial relations climate and HPWS. Findings: Union strategic influence has a positive effect on the take up of HPWS in unionised workplaces. Job security and the industrial relations climate demonstrate a serial mediation effect between union strategic influence and the take up of HPWS: union strategic influence has a positive effect on job security, which in turn positively impacts the industrial relations climate, thereby increasing the likelihood of the adoption of HPWS. The findings for the industrial relations climate are particularly strong. Practical implications: Findings suggest that organisations will benefit from focussing on the development of positive industrial relations, where unions have genuine strategic influence, because this maximises the likelihood that HPWS can be adopted and sustained. Originality/value: The paper provides a novel focus on the take up of HPWS within unionised workplaces. It focusses on the role of union strategic influence and the mediating effects of job security and the industrial relations climate, which are contextual factors that have been underexplored in the HPWS literature to date.