People form part of the biosphere - the biosphere being the whole intertwined network of life on Earth. While there is convergence on the need for societal change for just sustainability and a healthy biosphere, the pathways to achieve these transformations remain relatively unclear. Through legal interpretation, conceptual and thematic analysis of academic and grey literature, we seek to answer the following two questions: (a) how does the concept of Biosphere Defender enable a deeper and distinct understanding of who environmental defenders are and their role in just sustainability transformations? and (b) how does international human rights law contribute to specify the content of biosphere defenders’ rights? To address these questions, we first critically review the scope and limitations of the notion of Environmental Human Rights Defenders (EHRD) in human rights narratives in international law and policy. We examine how understandings of EHRD portray those defending their land and environment and what limitations this concept has in terms of possibilities for reflecting on transformations towards just sustainability. Second, we propose an alternative and/or complementary understanding of EHRD by using the concept of Biosphere Defenders. We also develop the Defend-Biosphere Framework to analyse the role of these actors as agents of change in pathways towards just sustainability. Third, to empirically illustrate the role of Biosphere Defenders, and the use of the Defend-Biosphere Framework we present two case studies from Latin America analyzing initiatives catalyzed by rural people who are defending their lands and territories while generating new ways to relate to socio-ecological systems and engage with the State and the economy. In both case studies, we find that relational values of solidarity, responsibility, and care (between human and other living beings) are central in understanding Biosphere Defenders’ initiatives creating pathways towards just sustainability. The findings of this article are of particular relevance to the implementation of the right to a clean, healthy and sustainable environment in the context of the Escazu Agreement on access to information, public participation and access to justice in environmental matters in Latin America and the Caribbean, the Aarhus Convention on access to information, public participation and access to justice in environmental matters and the Montreal-Kunming Global Biodiversity Framework in particular Target 22 (access to information, participation, access to justice and environmental defenders) and Target 23 (gender equality).