Active interactions among actors operating in the same space result in the facilitation of information and knowledge exchange. Researchers have intensively discussed how organizational proximity affects the diffusion of new social elements. From a social perspective, actors sharing the same class or status form meaningful relationships and interact as peer groups, which facilitates the diffusion of new technologies and practices among them. From an economic perspective, actors targeting the same consumer groups imitate one another to avoid falling behind in competition, resulting in the diffusion of similar products and services. While each perspective has proposed proximity and corresponding distance concepts among actors and discussed their outcomes, a comprehensive understanding of proximity that integrates these perspectives remains insufficient. This study distinguishes organizational proximity into social proximity and ecological proximity by type and discusses the mechanisms through which each type of proximity contributes to organizations' adoption of new institutions. The study also discusses the relationship between these two types of proximity and their interaction effects on organizational decision-making. To substantiate these discussions, panel data on the adoption of the admissions officer system by four-year comprehensive universities in South Korea were empirically analyzed, and all hypotheses were strongly supported. The results of this study contribute theoretically to proximity research while also providing policy implications for the diffusion of new institutions.