["Business Strategy and the Environment, EarlyView. ", "\nABSTRACT\nThis study investigates how open innovation is organized and coordinated when multiple actors work together in platform‐based circular economy (CE) ecosystems. This study also examines how these ecosystem interactions and partnership dynamics shape firms' ability to generate successful circular innovations and what conditions make these effects stronger. A two‐phase mixed‐method design is used. The first phase involves semistructured interviews with 20 European platform firms engaged in circular initiatives. The analysis identifies core governance mechanisms and partnership dynamics that shape collaboration in multi‐actor CE ecosystems. These insights inform the development of a conceptual model that explains how coordination practices, cross‐boundary negotiation, and credibility derived from partner affiliations interact to influence circular innovation outcomes. The second phase analyzes the measurement model using 400 survey data with covariance structural equation modeling and further tests the moderated‐mediation hypotheses using PROCESS. The findings contribute to the literature by providing empirical evidence that effective governance, cross‐boundary capabilities, and institutional legitimacy interact to shape innovation performance in CE ecosystems.\n"]