This study investigates how digital maturity models mediate cross-stakeholder alignment in healthcare digital transformation. While maturity models proliferate across healthcare contexts, existing research provides limited explanation of how they facilitate alignment among heterogeneous stakeholders in practice. Drawing on boundary object theory and an ethnographic case study of a national Health Big Data project involving research hospitals across four clinical networks, we examine how a digital maturity model enabled coordination among IT professionals, data managers, research coordinators, and administrative leaders spanning diverse organizational priorities and technical backgrounds. Our findings make three contributions: we reconceptualize digital maturity models as active coordination devices rather than passive diagnostic instruments; we extend boundary object theory by identifying three emergent properties crucial for mediating alignment-operational generativity, temporal scalability, and institutional anchoring; and we provide empirical insight into how maturity models shape sensemaking, negotiation, and coordinated action in federated healthcare ecosystems.