The imperative to “link knowledge and action” is widely invoked as a defining characteristic of sustainability research. The complexities of sustainability challenges such as climate change and biodiversity loss mean that linear models of knowledge and action, where knowledge is produced first (by researchers) then “applied to” action (by policy actors), are considered insufficient. Researchers have developed more dynamic, open-ended and collaborative forms of policy engagement such as transdisciplinary and co-production research. Although promising these approaches often remain captive to linear assumptions that hinder their transformative potential. We contribute by providing a relational model of knowledge and action rooted in contemporary practice theory. A practice-based approach suggests the primary task of participants in transdisciplinary interventions is to find workable solutions to situations of dynamic complexity that are fundamentally indeterminate and unpredictable. Knowledge is not “applied to” action, but drawn upon, produced and used from within the situation at hand, allowing researchers and policy actors alike to better harness the emergent character of situational developments and outcomes. A practice-based approach provides a conceptual language that captures the experienced complexities of intervening for sustainability, reconfigures the nature of “actionable knowledge,” and identifies appropriate modes of evaluation for transdisciplinary and co-produced research.