There has been a great deal of research on the relationship between the built environment and walking activity, but many studies have produced inconclusive or conflicting results. In this research we aim to enrich the association of the built environment with walking activity by examining its impact in Seoul, a Korean megacity characterized by high-density development and a well-equipped public transportation system. The results show that neighborhoods with a relatively higher land-use mix and relatively greater access to public transportation have a significantly positive association with walking activity for destinations that are within a 500 m radius of residences. However, no positive association was found between development density by land use and walking activity. Overall, the results of this study indicate that the relationship between the built environment and walking activity differs by neighborhood scale and the urban built environment in terms of density and public transportation across countries.