The timing of the rise of C4 grasses and the climatic conditions driving their expansion are widely debated, with recent studies suggesting that the expansion was more complex than previously thought. Proxy reconstructions from northwestern Argentina record multiple increases and decreases in C4 abundance throughout the late Miocene to early Pliocene, as well as variations in the timing of the spread of C4 grasses into the region. Here we present a multi-proxy paleovegetation reconstruction of the proportions of C3 and C4 vegetation using organic carbon and phytoliths preserved in paleosols at Entre Ríos in the Santa María Basin of Catamarca, Argentina between 6.2 and 3.3 Ma. Both δCorg13 and phytolith assemblages indicate a C3 dominated ecosystem with no evidence for substantial amounts of C4 vegetation from the late Miocene through mid-Pliocene, despite their rise to dominance elsewhere during this time. These data suggest that by 6.2 Ma the Santa María basin floor was at an elevation high enough to create locally cool conditions that allowed C3 vegetation to outcompete C4 grasses. Phytolith assemblages show that later uplift of the Sierra Calchaquies and Sierra Aconquija ranges through the Pliocene increased aridity and drove the expansion of C3 grasses at the expense of palms and other forest indicators. This multi-proxy paleovegetation reconstruction demonstrates that tectonics played an important role in controlling the late Miocene expansion C4 grasses by suppressing their spread into northwest Argentina.