Boundary spanner corruption—voluntary collaborative behavior between individuals representing different organizations that violates their organizations’ norms—is a serious problem in business-to-business (B2B) marketing relationships. Drawing on insights from the literatures on the dark side of business relationships and deviance in sales and service organizations, the authors identify boundary spanner corruption as a potential dark side complication inherent in close B2B marketing relationships. The same elements that generate benefits in interorganizational relationships, such as those between customer and seller firms, also enable the development of boundary-spanning social cocoons that can foment corrupt activities under certain conditions. A conceptual framework illustrates how trust at the interpersonal, intraorganizational, and interorganizational levels enables corrupt behaviors by allowing deviance-inducing factors stemming from the task environment or from the individual boundary spanner to manifest in boundary spanner corruption. Interpersonal trust between representatives of different organizations, interorganizational trust between these organizations, and intraorganizational agency trust of management in their representatives foster the development of a boundary-spanning social cocoon—a microculture that can inculcate deviant norms leading to corrupt behavior. Boundary spanner corruption imposes direct and opportunity costs on the involved organizations, with the additional burden of latent financial risk associated with potential exposure. The authors substantiate their multi-level framework and propositions with field-based insights from qualitative interviews with senior executives. The multi-level framework of boundary spanner corruption extends beyond extant marketing literature, highlights intriguing directions for future research, and offers new managerial insights.