<p>This study examines behavioural sustainability as the foundational mechanism linking individual, organisational, and societal change in circular economy transitions. While circularity is often approached through technology, policy, and business model redesign, the behavioural conditions that enable or constrain adoption and persistence remain underdeveloped. Using PRISMA-guided systematic review methods, the study synthesises peer-reviewed evidence to identify behavioural barriers, enablers, and intervention strategies across consumer, organisational, and governance contexts. Building on the domain-by-level synthesis, the review maps sector-specific behavioural constraints and leverage points and interprets them through a linear–recycling–circular transition lens. Findings show that circular initiatives frequently consolidate at recycling stages because intention–action gaps, friction costs, limited perceived control, and weak social legitimacy continue to favour linear defaults, while organisational capability constraints and misaligned incentives hinder transformation. The paper advances the Behavioural Sustainability as the Foundation of Circular Progress model, which positions behavioural mechanisms as the enabling base for transition progression. Drawing on the Theory of Planned Behaviour, Consumption Value Theory, and Value–Action Gap Theory, the model clarifies how attitudes, norms, perceived control, and value cues interact across levels to shape durable circular practice. The review concludes that circular transitions require integrated, multi-level intervention configurations aligning infrastructure, incentives, organisational routines, and policy signals to support sustained circular habits and progression beyond recycling.</p>