Nonprofits address some of the world's most pressing problems, and many rely on donations to fund their essential work. Nonprofit marketers are, therefore, tasked with promoting charitable giving. Research on giving has largely focused on identifying the characteristics that make people generous but has generated inconsistent findings, suggesting important moderators that are not well understood. Moreover, there is not yet an overarching theory to help integrate the vast and interdisciplinary literature. To address this, we propose Charitable Triad Theory, a new theory of giving with three key tenets: First, giving is triadic because the characteristics of three actors-donors, beneficiaries, and fundraisers-influence charitable decisions. Second, the characteristics of each of the three actors may be necessary but not sufficient to promote giving. Third, giving is relational because interactive relationships between the triad determine charitable choices. A systematic review of 1337 empirical articles published between 1980 and 2020 helps evidence seven key propositions of the theory, which articulate the ways in which donors, beneficiaries, fundraisers, and the dyadic and triadic relationships between them, can affect charitable behavior. We end with a research agenda outlining specific suggestions for future research on (a) the neglected fundraiser and (b) how beneficiaries influence giving.