Online influencer marketing (OIM) has become an integral component of brands’ marketing strategies; however, marketers lack an adequate understanding of its scope, effectiveness, and potential threats. To fill this gap, this article first describes the phenomenon’s background, defines OIM, and delineates its unique features to set the conceptual boundaries for the new concept. Drawing insights from practitioner and consumer interviews, and in line with social capital theory, the authors propose that OIM can be understood as leveraging influencer resources (including follower networks, personal positioning, communication content, and follower trust) to enhance a firm’s marketing communication effectiveness. Six novel propositions illustrate the benefits and potential threats of OIM, which may spur progress toward a theory of OIM. Finally, this article outlines key strategies for effectively managing OIM and identifies important literature–practice gaps to suggest avenues for further research.