This chapter analyses plant fossils including phytoliths, carbonized wood, and leaf compressions, along with microcharcoal, chrysophycean stomatocysts, sponge spicules, and diatoms, recovered from the Estancia La Costa Member, Santa Cruz Formation (late Early Miocene), in the Atlantic margin of Patagonia, Argentina. The floristic composition and paleoclimatic inferences based on this fossil plants assemblage from the late Early Miocene are presented. The fossil flora is characterized by the presence of herbaceous components including chloridoid, panicoid, danthonioid, pooid, and festucoid grasses. The arboreal elements include members of the Araucariaceae, Lauraceae, Arecaceae, Nothofagaceae, Myrtaceae, Cunoniaceae, Fabaceae (Faboideae), and ?Proteaceae. The vegetation was a mixture of open temperate semi-arid forests and humid warm-temperate forests. The integrated analysis of multiple sets of proxy data suggests that southeastern Patagonia, during late Early Miocene, was characterized by a temperate to warm-temperate and semi-arid to humid climate, where seasonal low levels of precipitation served as a limiting factor for plant growth.