The aim of this study was to examine whether the association between psychological capital (PsyCap; which is comprised of hope, self-efficacy, resilience, and optimism) and optimal engagement is mediated by positive emotions. Three studies were conducted across different cultural contexts (Hong Kong SAR, Mainland China, and the Philippines), domains (academic and work), and age groups (adult employees and school-aged adolescents). Study 1 was a cross-sectional study conducted among 162 employees in Mainland China. Study 2 was a cross-sectional study conducted among 71 secondary school students in Hong Kong SAR. Study 3 was a prospective study conducted among 404 secondary school students in the Philippines. Across the three studies, the positive effect of PsyCap on engagement (academic and work) was partially mediated by positive emotions. This study enriches the understanding of the processes through which PsyCap is associated with optimal outcomes. Theoretical and practical implications are discussed.