A rapidly growing body of literature has explored associations between urban design attributes, which are conducive to walking, and social capital. The current study aimed to build on the limitations of previous research. Specifically, this study estimated the associations between traditional and novel walkable built environment metrics and social capital among a sample of adults in Japan. Data (n = 1010) from a randomly selected cross-section of residents (40–69 years old) from two areas in Japan were included. Social capital was assessed by questionnaires. Several objective and perceived walkable built environment attributes were calculated. Covariate-adjusted multivariable linear regression models were used to estimate associations between neighborhood built attributes and the three social capital scores. Street connectivity was negatively associated with activities with neighbors (b = -0.21, 95% CI -0.31, -0.11). Perceived population density was negatively associated with all three social capital scores, including social cohesion, activities with neighbors, and social participation (b = -0.21, 95% CI -0.30, -0.11, b = -0.15, 95% CI -0.24, -0.06, and b = -0.16, 95% CI -0.29, -0.02, respectively). Traditional walkability and Walk Score® were negatively associated with activities with neighbors (b = -0.04, 95% CI -0.07, -0.00 and b = -0.09, 95% CI -0.15, -0.04, respectively). No significant associations were observed between perceived walkability and social capital scores. Space syntax walkability was negatively associated with social cohesion and activities with neighbors (b = -0.12, 95% CI -0.23, -0.01 and b = -0.11, 95% CI -0.21, -0.01, respectively). This study provided unique findings demonstrating that walkable built environments may not necessarily support social capital in ultrahigh-density Asian cities.