Network effects in payments technologies drive concentration, but interventions to increase choice risk market fragmentation. We study whether interoperability can resolve this dilemma, using data from India’s Unified Payments Interface and a major pre-existing digital wallet. When these platforms became interoperable, overall digital payment usage rose. This increase was driven by regions that were more fragmented across payment platforms ex ante, consistent with a model in which interoperability expands effective network size without requiring consolidation on a single platform. Model-based estimates imply that interoperability raised total digital payment usage by over 50 percent in the year after integration.