Ageism is a global challenge and a public health concern that the recent COVID-19 pandemic has further exacerbated. Existing research has mainly focused on individual factors, overlooking the association between the neighborhood-built environment and ageism. This study examined this association and whether its effect varied among areas possessing different socioeconomic characteristics. We conducted a cross-sectional survey of 1278 older people in Hong Kong and merged this with the built environment factors derived from geographical information system data. We used multivariable linear regression to examine the association. Findings revealed that more parks were significantly associated with a lower level of ageism, an impact that remained significant in low-income or low-education areas. Conversely, more libraries in high-income areas were associated with a lower level of ageism. Our findings provide urban planners and policymakers insight into planning for the built environment that reduces ageism enabling older people to achieve a better life.