From the Upper Cretaceous (Maastrichtian-Campanian) Huepac Chert Locality of the Tarahumara Formation, a new extinct aquatic plant of the Haloragaceae (Subclass: Rosidae), Tarahumara sophiae Hernández-Castillo and Cevallos-Ferriz, is presented. It is reconstructed on the basis of both reproductive and vegetative organs preserved in chert. Its description and comparison with extant plants are based on the analysis of 350 permanent slides made with thin section and peel techniques. A mosaic of anatomical and morphological characters found in the extant Myriophyllum, Meziella, and Haloragodendron is used to characterize the new taxon. The development from flower to fruit in the fossil plant follows similar stages found in some extant species of Myriophyllum. However, a floral cup is reported for the first time in the family, and it is compared to the persistent fused sepals (terminal corona) of Meziella. This plant reconstruction increases the morphological diversity of the Haloragaceae, demonstrates the presence of a new dispersal mechanism compared with those known for the family, suggests that this lineage had originated by at least the Upper Cretaceous, and refutes the interpretation of several haloragacean Tertiary reports as representing genera of extant plants.