Seeds have become one of the most contested resources in our society. Control over seeds has intensified under neoliberalism, and today four large multinational corporations control approximately 70 percent of the global seed market. In response to this concentration of corporate power, an international social movement has emerged around the concept of seed sovereignty, which reclaims seeds and biodiversity as commons and public goods. This study examines the relationship between the global dynamics of commodification and enclosure of seeds, and the seed sovereignty countermovement for decommodification. I approach this analysis through an ethnographic case study of one local seed sovereignty movement, in the indigenous central region of Chiapas, in southern Mexico. I spent eight months between 2015 and 2016 conducting field research and documenting the development of the Guardians of Mother Earth and Seeds project, a local initiative focused on seed and food sovereignty that was initiated in 2015 by DESMI, the most established NGO working in this region. It encompasses 25 peasant communities--22 indigenous and 3 mestizo--from the Los Altos, Norte-Tulijá, and Los Llanos regions of Chiapas. I also collected data from 31 other communities in the region involved to varying degrees with this agenda of seed and food sovereignty. This study incorporates both communities affiliated with the Zapatista Army of National Liberation (EZLN) and non-Zapatista communities. Three research questions guide this dissertation: (1) How do the increasing industrialization and commodification of seed systems and agriculture affect peasant communities in Chiapas?; (2) How is the local seed and food sovereignty countermovement responding to those processes of commodification?; and (3) How does this case study contribute to understanding the relationship between capital's tendency to enclose the commons and the protective countermovements that attempt to resist such market encroachments? This study found that the development of industrial agriculture and the commodification of seeds at the global and national scales have implied neither the displacement of these communities' native seeds by commercial seeds, nor their privatization--two of the most frequent potential risks denounced by representatives of the national and international seed sovereignty movement. Instead, the main impact of industrial agriculture and Green Revolution policies in the study region has been the chemicalization of peasant agriculture, with attendant negative impacts on the environment and human health. I also found that subsistence agriculture--the main mechanism through which native seeds are reproduced within communities--is undergoing a process of severe deterioration, which partially responds to the neoliberal dismantling of governmental institutions and programs supporting peasant agriculture. A key finding of this research is that the deterioration of subsistence agriculture is the main risk that the neoliberal restructuring of agriculture poses to native seeds. In response to these developments, communities in this study have embraced a project of decommodification focused on enhancing and expanding their subsistence agriculture. This project encompasses agroecology, food production collectives, and initiatives for agro-biodiversity conservation and ecological restoration. I argue that this project contributes to the decommodification of subsistence agriculture in the region, primarily by strengthening the non-commodified structures that are essential for these communities social reproduction.