Flow is an optimal state of absorption that may be experienced in appropriately challenging and intrinsically motivating activities such as sports. Flow may be an important concept for understanding the emergence and role of sport in society, yet theoretical explanations of flow have had limited success explaining, predicting, and facilitating flow in sport. Here, we use the ecological dynamics framework, seeking to resolve foundational issues in an explanation of flow, building towards a theory of flow in sport. To address this challenge, we highlight the utility of ecological conceptualisations of experience, intention, skill, attention, information, and temporality, in explanations of flow experiences in sport, before discussing some novel empirical predictions motivated by the theory. We suggest that a multiscale ecological dynamics approach is well equipped to explore flow in performer-environment systems that display interaction-dominant dynamics and conclude by outlining avenues for future research created by an ecological dynamics theory of flow in sport.