Legitimacy is at the heart of knowledge politics surrounding agriculture and food. When people accept industrial food practices as credible and authoritative, they are consenting to their use and existence. With their thick legitimacy, industrial food systems paralyze the growth of alternative agricultures, including agroecology. Questions of how alternative agricultures can attain their own thick legitimacy in order to compete with, and displace, that of industrial food have not yet attracted much scrutiny. We show that both agroecological and scientific legitimacy grow out of a web of legitimation processes in the scientific, policy, political, legal, practice, and civic arenas. Crucially, legitimation often comes through meeting what we call ‘credibility tests’. Agroecologists can learn to navigate these co-constituted, multiple bases of legitimacy by paying attention to how credibility tests are currently being set in each arena, and beginning to recalibrate these tests to open more room for agroecology. Using a schematic of three non-exclusive pathways, we explore some possible practical interventions that agroecologists and other advocates of alternative agricultures could take. These pathways include: leveraging, while also reshaping, the existing standards and practices of science; extending influence into policy, legal, practical, and civic arenas; and centering attention on the ethical legitimacy of food systems. We conclude that agroecologists can benefit from considering how to build legitimacy for their work.