Our paper investigates the interaction of mechanisms that drive the emergence and development of a sustainable entrepreneurship ecosystem (SEE). We study a specific geographic context, the Amsterdam metropolitan area, and a specific industrial context, the denim industry. We conducted a qualitative, inductive study to understand how SEEs emerge and develop. We used data from longitudinal observations, semi-structured interviews and archival documentation to examine the emergence and development of the Amsterdam SEE in the denim industry. We show that the local culture that supports sustainability values is essential to attracting sustainability support organisations and acts as a catalyst for growth. By focusing on a specific sustainable innovation project as a case study, we also find that collectively engaging in innovation projects spurs the growth of an SEE and aids legitimization of SEE actors. Our study provides contributions to the nascent branch of literature on sustainable entrepreneurship ecosystems and has implications for practitioners and policy makers.