Diversification into new services and regions increases organizational visibility and survival. While the extant literature has provided insights into the determinants of diversification, it has not considered the relationship between an organization’s strategy to acquire resources and its diversification. We argue that the way in which organizations acquire their resources affects the efficiency of the resource acquisition process, which impacts organizational diversification into new services and geographic regions. More specifically, we propose that a higher number of resource providers and a shorter relationship between the organization and its resource providers make organizations less likely to diversify. We examine the effect of these two determinants—number of providers and relationship duration—on diversification empirically in the context of institutional donors that fund humanitarian organizations, using more than 150,000 donations over 20 years. Our results give support to our arguments; we demonstrate that relying on a large number of resource providers constraints diversification while relying on long-term relationships is positively related to diversification.