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Despite significant clinical progress, checkpoint blockade remains limited by variable response rates, resistance, and toxicity. Activating costimulatory receptors offers a promising alternative to enhance anti-tumor immunity. However, there is insufficient understanding of how to mimic physiological membrane-anchored costimulatory ligands. Here, we describe a strategy for developing effective agonists of the costimulatory receptor CD27 by increasing both antibody valency and FcγRIIB engagement. Engineered anti-CD27 antibodies capable of tetravalent binding to CD27 and selective FcγRIIB association exhibit potent T cell stimulatory activity and anti-tumor efficacy in pre-clinical models, compared to bivalent counterparts. The anti-tumor effects of the tetravalent antibody are mediated through CD8⁺ T cell activation without evidence of regulatory T cell depletion. Mechanistically, whereas the increase in avidity drives more efficient CD27 clustering, FcγRIIB engagement triggers polarization of receptor clusters to the cell-cell interface and reduces receptor internalization. This work provides a framework for developing more effective agonist-based T cell stimulatory therapies.