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The spread of grasslands in the Miocene and of C4 grasses in the late Miocene-Pliocene represents a major development in terrestrial plant evolution that affected the climate system and faunal evolution. The macrofossil record of grasses is sparse, likely due to the limited preservation potential of grasses. Diagnosis of the C3 or C4 photosynthetic pathway depends on preservation of both cellular structures and organic carbon for isotope analysis. Here we analyze the anatomical and isotopic composition of newly-collected grass fossils from the Dove Spring Formation, Red Rock Canyon State Park, California, USA, located in the El Paso Basin on the western side of the Basin and Range Province, a site previously identified as one of the earliest known C4 grass fossil bearing localities. We analyzed the anatomical and geochemical characteristics of these new grass fossils dated to 12.01–12.15 Ma. The fossils analyzed in this study include grass shoots and in cross-section display anatomy indicative of the C3 photosynthetic pathway. We isolated organic carbon from the stem fossils and determined the carbon isotopic composition to be − 24.8 ± 0.5‰. Together, the anatomical and geochemical analyses confirm that these plants used the C3 photosynthetic pathway. Our findings are consistent with dietary evidence based on tooth enamel from grazing mammals of available C3 resources in the same sections. These newly reported Miocene-age C3 grass fossils contribute to a sparse macrofossil record of grass evolution. Overall, paleoecological reconstructions at this site indicate more humid conditions during the Miocene compared to the modern Mojave Desert with C3 grasses and diverse grazing mammals.