The aims of the study are to investigate the relationship between density and social equity. Social equity is an important social goal with regard to urban development, especially smart growth and sustainable development; however, a definition of the concept of social equity from an urban planning perspective was still lacking. In response to these deficiencies, the study used quantitative and qualitative methods and synthesized multiple social and spatial perspectives to provide guidance for density and social equity planning, community design, and public policy. This study used data for the area of King County, Washington to explore the empirical relationship between density and social equity at the neighborhood level. In examining access to several facilities, this study found that distances to parks and grocery stores were shorter than those to other facilities, such as the library, hospital, police station, and fire station. In terms of the relationship between density and accessibility, the results show that higher density is associated with better accessibility in neighborhoods. Density is also positively associated with both income diversity and affordable housing for low-income families. In terms of the relationship between density and crime, density is positively associated with violent crime, while density is negatively associated with property crime. The findings of this study can aid in the development and evaluation of urban policy and density planning aimed at promoting social benefits in urban space. Therefore, this study is useful to a range of stakeholders, including urban planners, policy makers, residents, and social science researchers across different disciplines.