<p>This study systematically reviews how established entrepreneurial ecosystems (EEs) enable and constrain circular startups as a distinct form of circular entrepreneurship. Analyzing 61 peer-reviewed articles, the review identifies five themes capturing how core EE elements function for circular startups: (1) resource endowments, revealing the amplified ecosystem dependency of circular startups and the need for a reconfiguration of resources; (2) collaboration, emphasizing multi-actor partnerships and the orchestration of actors across value chains within networks; (3) culture, highlighting trust and shared values as enablers of collective action; (4) regulatory framework, outlining adaptive and transformative governance and policy mechanisms that align entrepreneurship with circular objectives; and (5) digital technologies, which enhance transparency, resource circulation, and coordination. Together, these clusters form a conceptual understanding of how EEs transform into circular entrepreneurial ecosystems (CEE). Focusing on circular startups, the study delineates how established EE elements must be reconfigured and adapted to support startups that embed circularity from scratch. Building on the identified gaps, it proposes a future research agenda that calls for consolidation of CEEs, deeper inquiry into resource dynamics and intermediation, and multi-actor approaches that integrate cultural, policy, and digital dimensions. The study advances the concept of EE by explaining how the effectiveness of ecosystem elements depends on their adaptation to circular principles and provides a theoretical foundation for designing EEs that foster circular entrepreneurship.</p>