Since the Enlightenment, humanist philosophy has understood materiality as an inert and determinate world categorically separate from the sphere of consciousness and language. However, after evolving significantly during the 20th century, the natural sciences now recognize the complexity, indeterminacy and agency of matter. A parallel transformation can be observed in contemporary Spanish and Latin American literature and is exemplified in the works of Cristina Peri Rossi and Cecilia Vicuña. Drawing on knowledge which emerges from the natural sciences, the humanities and personal experience, these poets explore multiple dimensions of materiality from the microscopic world of subatomic particles and DNA molecules to the macroscopic world of the body and the structure of the universe. The theoretical orientation of this study emerges from posthumanism, which critiques the epistemological foundations of humanist thought and reconfigures reductionist concepts of matter, discourse, the subject, and agency which are grounded in dualistic ontology. Material feminist theorists explore materiality through interdisciplinary approaches which establish a dialogue between posthumanism, feminist theory and the natural sciences. The material feminist Karen Barad proposes an agential realist ontology which constitutes the principal theoretical framework of this thesis. According to Barad, phenomena are not exclusively social or material but rather material-discursive practices, and the concept of agency is reconfigured as the product of the dynamics of intra-action rather than an as an attribute restricted to the human sphere. Furthermore, this thesis utilizes diverse materials from the areas of literary criticism and scientific research in order to achieve an authentically interdisciplinary interpretation of materiality in the poetry. Peri Rossi and Vicuña express a profound questioning of the fundamental assumptions of humanism and offer perspectives which take into account matter's agency and dynamism. Their poetry presents materiality as a constant process of creation and as an active participant in the unfolding of reality, thereby opening up new horizons of investigation. By interpreting the works of Peri Rossi and Vicuña through the lens of posthumanist theory, this study contributes to a growing body of interdisciplinary approaches to contemporary Spanish and Latin American literature.