Human resource management (HRM) is a major mechanism to realise decent work and achieve employee well-being in China. Key questions concern how and when HRM is mutually beneficial for employers and employees, particularly how well HRM works with trade unions in achieving decent work. This study examines the effect of high-performance work systems and trade unions on employee engagement, paying particular attention to the role of industrial relations (IR) climate in facilitating this relationship. Hypotheses were tested via unique cross-sectional survey data from a large employer–employee matched dataset collected from 264 manufacturing firms and 6921 workers in China. The findings highlight that HPWS are a key mechanism for realising decent work. We also find positive evidence for the mutual gains of decent work illustrating the combined impact of HPWS and trade unions in achieving higher levels of employee engagement. Notably, a co-operative (as opposed to adversarial) IR climate is seen to reinforce this relationship, indicating the significance of the quality of relationships among management, employees, and unions in shaping employee outcomes. Overall, these findings advance understanding of the realisation of decent work and ‘mutual gains’ in a Chinese context.