Interrelationships between Catastrophic Health Expenditure, Social Capital, and Psychological Distress among Retirees: A Scoping Review Introduction Global population ageing heightens financial and psychological vulnerabilities among retirees. Catastrophic health expenditure (CHE), when out-of-pocket spending surpasses household resources, threatens financial security, while psychological distress remains common in later life. Social capital, comprising networks, trust, reciprocity, and community engagement, has been identified as a potential buffer against both financial hardship and mental health decline. However, the intersection between CHE, social capital, and psychological distress in retirees has not been comprehensively mapped. Methods This scoping review compiled global empirical evidence on the interrelationships among these three constructs and identified research gaps relevant to healthy ageing policy. The review followed the Arksey and O’Malley framework, enhanced by Levac et al. and reported according to PRISMA-ScR. Searches across PubMed, Scopus, and Web of Science covered English-language studies from 2000 to 2025. Results From 1,746 records, 20 studies met the inclusion criteria. Most originated from Asia, with others from Africa, Europe, and the Americas. CHE was associated with greater psychological distress in nine studies, while six studies showed that stronger social capital corresponded to better mental well-being. One study found that social capital mitigated the psychological impact of CHE. Integrated analyses covering all three constructs remain limited. Conclusions Strengthening both financial protection and social support systems may help safeguard retirees’ mental health, while further longitudinal and cross-cultural research is needed.