Industrial-Urban Symbiosis (I-US) lies at the heart of the Theseus Hub for Circularity (H4C), fostering cooperation between municipalities, regions, and industries in managing resources, waste, energy, water, infrastructures, and networks. This first-of-a-kind initiative in Greece, originating from the Athens/Attica metropolitan area, aims to lead the nation in establishing a robust circular economy model aligned with European strategies under the Processes4Planet (P4P) partnership. The Theseus H4C will validate and implement cutting-edge solutions across water, energy, and materials, leveraging digital and robotic technologies to address regional needs. Its systemic approach integrates innovative governance models and collaboration frameworks, enabling pilot projects, scaling up solutions, and expanding a dynamic ecosystem. By targeting widely impactful material, energy, and water flows, the hub ensures its methodologies can be replicated in other European regions, contributing to climate neutrality by 2050. This ambitious initiative, inspired by the mythical figure of Theseus, positions Athens as a leader in sustainable reform and urban transformation, while creating a blueprint for eco-industrial synergy across Europe.