The transition toward a circular economy poses substantial challenges for manufacturers, who often lack actionable artifacts to assess their readiness and prioritize transformation efforts. While existing circular economy maturity models and digital sustainability tools offer conceptual guidance, they frequently remain generic or weakly connected to organizational decision-making. This study develops a Circular Transition Assessment Tool (CTAT) tailored to the plastics industry, which is notorious for its linear production philosophy. We adopt the design science research approach to develop a design theory: meta-requirements, design requirements, design principles, and design features for a circularity self-assessment system. Subsequently, we translate the design theory into an instantiated artifact. CTAT provides multicategorial, qualitative, and quantitative maturity feedback, contextual explanations, and actionable recommendations supported by transparent assessment logic and expert validation. The artifact is iteratively evaluated through workshops with plastics manufacturers and industry experts. The results indicate that CTAT helps organizations reflect on their current state of circularity, obtain credible assessments, and make informed decisions. The study contributes to the cumulative design knowledge of sociotechnical systems for sustainability and demonstrates how industry-specific assessment systems can support circular economy transitions in complex industrial contexts.