 Amargasaurus is classified as a member of the Dicraeosauridae, a family ranked clade within the Diplodocoidea. Currently, this clade consists of five species belonging to four genera. These are, besides Amargasaurus cazaui, the species Dicraeosaurus hansemanni and Dicraeosaurus sattleri from the Late Jurassic Tendaguru beds of Tanzania, and the recently described Brachytrachelopan mesai from the Late Jurassic of Argentina. Whitlock (2011) argued that Suuwassea emilieae from the Morrison Formation of the United States has to be placed inside the Dicraeosauridae as well, which was supported by subsequent studies. Amargasaurus is the only named dicraeosaurid from the Cretaceous; however, an unnamed specimen from Brazil indicates that this group persisted until the end of the Early Cretaceous. Most analyses find Dicraeosaurus and Brachytrachelopan to be more closely related to each other than to Amargasaurus. Suuwassea was recovered as the most basal member of the family. A recent analysis by Tschopp et al. (2015) came to the preliminary result that two poorly known genera from the Morrison Formation, Dyslocosaurus polyonychius and Dystrophaeus viaemalae, might be additional members of the Dicraeosauridae.
