Are unions helpful in companies that extensively invest in high-performance work systems (HPWS)? The current literature does not offer a conclusive answer to this question as studies provide contrasting views. Extending from prior HPWS studies, this study theorizes and examines the interactive effect of HPWS and union presence on firm performance and employee well-being and withdrawal. Using longitudinal data from South Korea with 935 firm-level observations, it was found that HPWS had a dual-path effect on firm performance and employee well-being and withdrawal via parallel mediations of cooperative and performance climates. The overall impacts of HPWS on firm performance and employee outcomes were more positive in unionized firms than in non-unionized counterparts. This study responds to prior studies’ call and offers a more comprehensive understanding of HPWS’ nuanced relationships with firm performance and employee well-being to the literature.