Established firms are increasingly forming alliances with green start-ups, i.e. newly-started ventures dedicated to eco-innovation. This study explores the rationales for the initiation and continuation of such alliances. We investigate how established firms leverage these alliances to facilitate eco-innovation adoption alongside their pursuit of legitimacy. Findings from a case study on seven alliances focused on food waste reduction initiatives in Italy and Sweden indicate that legitimacy-seeking rationales are predominant during alliance formation. As alliances evolve, they transition into a phase where firms identify opportunities for eco-innovation, while ambitions of capability-building and new revenue creation shape the development of the alliances. This study contributes to eco-innovation and environmental alliance literature by showcasing how the pursuit of legitimacy facilitates established firms’ adoption and implementation of eco-innovation through alliances with green start-ups.