This study investigates the theoretical causal relationships among neighborhood built environments, social capital and social sustainability using structural equation modeling (SEM), through a case study in Seoul, Korea. The dataset consisted of responses from a questionnaire survey completed by 500 respondents. Neighborhood built environments were also objectively measured by GIS analysis, using a 250-m buffer based on the home addresses of the respondents. A total of four latent variables of the neighborhood physical environments were used in the model: perceived neighborhood environment, characteristics of the residential area, land use diversity and accessibility to parks and sport facilities. Respondents' demographic and socioeconomic characteristics were also considered in the model. The results of the analysis indicate that there is a statistically significant causal relationship among neighborhood physical environment, social capital and social sustainability. The results also suggest that neighborhood-level spatial and non-spatial factors can influence the formation of social capital that affects social sustainability. Moreover, this result indicates the possibility that urban spatial planning can play a critical role in social issues.