<p>For older adults, the impact of the built environment, including housing conditions and neighborhood infrastructures, is particularly significant since they often spend a lot of time at home or in their local area. Limited social networks, interactions, and physical limitations contribute to isolation, more so for older adults living alone. Thus, this study aims to investigate the role of social capital in the context of the built environment, particularly focusing on substandard housing environments and their association with depression among older adults living alone. The study employed a quantitative research method with a sample consisting of 1557 older adults living alone in Korea, drawn from the 16th wave (2021) of the Korean Welfare Panel Study. The outcome variables examined were depressive symptoms (CESD-11), built environment, and social capital. Structural equation modeling (SEM) was conducted to test the latent interaction model involving social capital, built environment (substandard housing environments), and depression. Results from the SEM analysis indicated significant impacts of social capital and built environment on depression. Specifically, substandard housing environment showed a positive relationship with depression, while social capital demonstrated a negative relationship with depression. Furthermore, older adults living alone exhibited increased vulnerability to depressive symptoms due to their substandard housing environment, especially for those with low social capital. This study fills that gap by highlighting the importance of built environment—housing quality—and its interplay with social capital in influencing older adults’ health outcomes. Results indicated that social capital mitigates depression and attenuated the adverse effects of substandard housing environments on depression. Therefore, policy and practice interventions could prioritize efforts to promote social capital among older adults.</p>