<p>Social isolation is a known risk factor that can impact both physical and psychological well-being. Given that social isolation is inherently shaped by the presence, absence, and frequency of social interactions within the broader community, understanding the role of community-level factors is essential. However, there remains a notable gap in the literature regarding how these factors contribute to social isolation among young adults. This study explores the relationship between district-level capitals—economic, cultural, and social—and social isolation among young adults in Seoul, South Korea, while also examining how these associations differ between single-person and multi-person households. Data were drawn from the 2022 Survey on the Status of Isolated and Reclusive Youth in Seoul, encompassing 4,045 unmarried young adults (aged 19–39) across 25 districts. Multilevel modeling was employed to analyze the association between district-level capitals and social isolation. Higher proportions of college graduates were associated with lower levels of social isolation, whereas district-level social trust had no significant effect. Young adults living alone were more vulnerable to isolation, particularly in affluent districts with fewer low-income residents. The cross-level interaction analysis further revealed that the association between cultural capital and social isolation was significant only among multi-person households. Findings highlight the need for district-specific interventions to tackle social isolation among young adults, with a particular focus on single-person households.</p>