Abstract Resulting from the digital revolution of the last decades, multiple startup hubs flourished across the globe in the past 10 years. Healthy environments for the development of innovative, nascent digital enterprises require a well-balanced variety of agents and supporting processes, which we collectively call a software startup ecosystem. These ecosystems are fundamental for the insertion of countries in the digital economy of the twenty-first century. However, having all the elements that compose such environments in the most advanced and prolific state is difficult and relatively rare. In this paper, we show that startup ecosystems can evolve over time passing through a sequence of maturity level stages. For that, we introduce a maturity model for software startup ecosystems based on systematic qualitative research around a multiple case study we conducted across three ecosystems. The study was carried out over 4 years and included an extensive array of data collection mechanisms such as literature reviews, expert interviews, and observations in three relevant ecosystems (Tel-Aviv, São Paulo, and New York); all collected data were analyzed with techniques based on Grounded Theory, resulting in a conceptual framework of software startup ecosystems. Finally, we developed a maturity model for startup ecosystems, which helps us understand their evolution and dynamics. Moreover, it can serve as a basis for stakeholders in less mature ecosystems to analyze their environment, identify weak spots, and propose policies and practical actions for improving their ecosystems over time.