This study examines the correlations between urban environmental satisfaction, neighborhood relations, and livability. Previous studies on livability have insufficiently dealt with urban environments and neighborhood relations and have failed to conduct an integrated analysis that considers the causal relationships between these factors. To fill these knowledge gaps, this study includes urban environmental satisfaction and neighborhood relations as factors affecting livability. Moreover, this study verified the mediating effect of neighborhood relations between urban environmental satisfaction and livability. Online surveys were carried out with 750 residents in Seoul, South Korea, and the structural equation model (SEM) was employed. The results indicated that a higher level of urban environmental satisfaction affected livability positively. In particular, the accessibility had the greatest effect on livability. In addition, neighborhood relations had a mediating effect on the pleasantness and safety of urban environments. Today, many developing countries are undergoing rapid urbanization, as Seoul has experienced in the past. However, this can cause a number of simultaneous side effects, which lower livability. Furthermore, this leads to population decline which might hinder urban sustainability. Therefore, this study suggests important policy implications for achieving urban sustainability by improving livability.