The paper introduces the concept of ‘eco-digital constitutionalism’ to denote one of the latest offsprings of digital constitutionalism that, building on environmental constitutionalism, articulates rights and principles to face the ecological challenges of the digital revolution. We propose this concept as an analytical lens synthesising digital and environmental dimensions to study the response to the specific constitutional challenges of the twin transition: a convergence often overlooked by environmental or digital constitutionalism in isolation. In particular, the article leverages this new concept to investigate the emergence of digital sustainability principles within the EU, positioning the Declaration on Digital Rights and Principles as a catalytic normative blueprint. To contrast the Declaration’s fragmented structure, we develop a typology of three digital sustainability principles. We employ this categorisation as a parameter to conduct a socio-legal study of constitutionalisation of digital sustainability principles across EU member states. By examining a spectrum of sources, including constitutions, ordinary laws, policy documents and digital charters, our empirical findings reveal a process in its infancy, with no formal constitutional recognition and relegated to isolated pieces of legislation. On the contrary, policy documents remain the laboratory of digital sustainability principles, and digital charters display an effort to shift from principles to rights.