This paper examines the transformation of classical American naturalism into what may be termed Anthropocene naturalism, a revised literary mode shaped by climate crisis, ecological precarity, and posthuman vulnerability. While classical naturalism, as theorized by Émile Zola and later American critics, emphasized heredity, environment, and social forces as determinants of human life, contemporary American fiction relocates determinism within planetary and material systems that exceed human control. Drawing on Anthropocene theory, posthumanism, and new materialism, this study analyzes five key texts: Jesmyn Ward’s Salvage the Bones and Tom Piazza’s City of Refuge, which depict climate disaster and ecological determinism through Hurricane Katrina; Don DeLillo’s Falling Man, which foregrounds urban vulnerability and infrastructural fragility in the Anthropocene city; and Louise Erdrich’s The Round House alongside Ward’s Sing, Unburied, Sing, which engage land, law, racial ecology, and posthuman ethics. The paper argues that these novels revise naturalism by decentering human agency and emphasizing non-human forces, shared vulnerability, and environmental injustice. In doing so, the study contributes to ecocriticism, posthuman studies, and contemporary American literary scholarship by demonstrating the continued relevance of naturalism in an era of planetary crisis. This study employs comparative close reading informed by posthumanist theory, new materialism, and Anthropocene studies to examine how determinism is reconfigured in contemporary American fiction.