Much debate exists over the taxonomic relationships among the river dolphins, which have traditionally been grouped at the superfamily level (Platanistoidea) or at the family level (Platanistidae), but monophyly of this group has been questioned by several recent studies. The present study uses endocasts from the river dolphin taxa and a variety of marine taxa in order to investigate this problem further. Endocasts from crania belonging to the river dolphins (Inia geoffrensis, Pontoporia blainvillei, Lipotes vexillifer, and Platanista spp.) were used to described various features of the brain and dural venous sinuses. Casts from Inia, Lipotes, and Pontoporia had large projections from the ethmoid region and Platanista unexpectedly had large projections from the optic nerve region. Overall patterns of dural venous sinuses were similar among all taxa for the dorsal sagittal sinus, the confluens of sinuses, and the transverse sinuses. The spinal venous plexus and longitudinal, basilar, and dorsal petrosal sinuses were highly variable among taxa. Based on the results of this study, we advocate conclusions reached by previous authors who have placed Inia and Pontoporia into one group, with Lipotes as a sister taxon and Platanista as more closely allied with the delphinids.