This paper examines how entrepreneurial opportunities co-evolve within a sustainable entrepreneurship ecosystem (SEE). Most of the literature on entrepreneurial ecosystems falls short on integrating the entrepreneurial process in empirical research. To analyze data collected from pre-start-up teams within a nascent SEE on high-tech cellulose-based materials over 3 years, we apply a design science approach that helps understand actors’ collaborative sensemaking in designing and structuring ecosystem features and relationships. Our findings show that the SEE can be seen as a design artifact which evolves by ecosystem actors collectively engaging in new venture ideation and developing opportunity confidence. Furthermore, the paper presents a novel SEE framework, which elaborates on phases and enablers of the opportunity co-evolution process within an emerging ecosystem. We contribute to the literature on sustainable as well as general entrepreneurial ecosystems and offer a new theoretical foundation for a process view on ecosystems.