<p>The concept of sustainability has fostered increased interest among service researchers, particularly in relation to making service ecosystems more sustainable. While this growing body of work has examined relevant drivers and barriers and explored strategies for facilitating such transitions, the concept of sustainable resource integration remains underdeveloped. With no clear definition to guide research and practice, the need for theorizing resource integration from a sustainability perspective is key. To address this gap, the present study employed a theory adaptation approach, drawing on service-dominant logic (domain theory) and sustainability and circular economy (method theories) to define sustainable resource integration as using resources in ways that are materially supportable, performatively viable, and temporally adaptive, such that the activities can be continued without undermining the systemic conditions it depends upon. In addition to a definition, the paper provides three propositions as a foundation for theorizing sustainable resource integration and its role in the evolution of service ecosystems.</p>