["Business Ethics, the Environment &Responsibility, EarlyView. ", "\nABSTRACT\nBusiness‐nonprofit collaborations are essential for addressing organizational and societal challenges, yet research has largely examined their formation from the perspective of a single partner rather than as a two‐sided process. In this study, we conceptualize collaboration formation as a dyadic matching process and examine two related decisions: whether a business and a nonprofit select each other as partners and what type of collaboration they form. Drawing on Transaction Cost Economics and the Resource‐Based View, we analyze a novel dataset of 24,190 Italian business–nonprofit dyads using Probit, Multinomial Probit, and Heckman‐corrected Probit models. The results show that firms and nonprofits select each other as partners when they are geographically proximate and more symmetric, consistent with lower transaction costs in partner search, negotiation, and monitoring. However, these conditions—especially symmetry—are less important in explaining more intensive collaborations characterized by bidirectional resource exchange. These collaborations are instead more strongly associated with resource similarity between partners, whereas philanthropic collaborations are more likely when partners rely on dissimilar resources. Our study enriches the literature on business‐nonprofit collaborations, advancing the understanding of their heterogeneous and bilateral nature, and offers implications for practitioners and policymakers interested in collaborative sustainability strategies.\n"]