The genus was named in 1927 by Friedrich von Huene but without a description, so that it remained a nomen nudum. In 1929 the type species, Laplatasaurus aurakanicus, was described by von Huene. The generic name refers to La Plata. The specific name is derived from the Auracanos or Mapuche. By accident von Huene in 1929 also mentioned a "Laplatasaurus wichmannianus" but that was a lapsus calami for Antarctosaurus wichmannianus. In 1933 however, he and Charles Alfred Matley renamed Titanosaurus madagascariensis to Laplatasaurus madagascariensis. This last species is today commonly referred to the original Titanosaurus.
Von Huene based Laplatasaurus on fragmentary material found in three locations in Argentina, in strata of the Allen Formation, dating from the Campanian faunal stage. It consisted of limb elements, some dorsal vertebrae and a series of caudal vertebrae. Part of the finds had earlier been referred by Richard Lydekker to Titanosaurus australis. Von Huene never assigned a holotype but in 1979 José Fernando Bonaparte chose MLP 26-306 as the lectotype, a specimen consisting of a tibia and a fibula that perhaps originate from different individuals.
Von Huene assigned those fossils to Laplatasaurus that seemed to indicate a rather large yet at the same time elegantly built sauropod. The about eighteen metres (60 ft) long Laplatasaurus was perhaps similar to Saltasaurus. Osteoderms forming an armored plating on the back, have been referred to Laplatasaurus but the association is uncertain. These plates had much smaller ridges than those of Saltasaurus.
Von Huene placed Laplatasaurus in the Titanosauridae, which is still a common classification. In 1986 Jaime Eduardo Powell mentioned the species as a Titanosaurus aurakanicus, and in 2003 explicitly proposed this combination as a replacement name. Others however, see Laplatasaurus as valid genus.