The genus contains a single species known from at least six young adult individuals, Patagotitan mayorum, which was first announced in 2014 and then named in 2017 by José Carballido and colleagues.

It can be distinguished from its close relatives by a suite of unique characteristics in its back and tail vertebrae, scapulae and humeri in the forelimb, and ischia and femora in the hindlimb.

A part of a lower femur was discovered in 2010 by a farm laborer, Aurelio Hernández, in the desert near La Flecha, Argentina, about 250 km (160 mi) west of Trelew.

[1] The collected sauropod fossils were organized into six partial skeletons, which likely belonged to the same species due to uniformity in their morphology and size (with all individuals differing no more than 5% in length).

Within the 3.43-metre (11.3 ft) sediment layer containing the fossils, there are three distinct but closely-spaced levels corresponding to three burial events, which are named FLV1, FLV2, and FLV3.

Another quarry was excavated about 300 metres (980 ft) west, revealing a fourth sediment layer (FLV4) with a similarly-sized sauropod skeleton.

[1][2] In 2017, José Luis Carballido, Diego Pol, Alejandro Otero, Ignacio Alejandro Cerda, Leonardo Salgado, Alberto Carlos Garrido, Jahandar Ramezani, Néstor Ruben Cúneo and Javier Marcelo Krause named these remains as the type species of a new genus, Patagotitan mayorum.

[1] Following the initial publication of Patagotitan in 2017, science writer Riley Black and paleontologist Matt Wedel cautioned against the reported estimates.

[13][14] Wedel also criticised the polygon-based method that Carballido and colleagues used to compare the sizes of Patagotitan and Argentinosaurus' vertebrae, noting that the former was largely empty space.

Also, he criticized the 3D model used for the volumetric estimate as having a flat-sided torso, and noted that the neck and tail were reconstructed as being fairly long.

Uniquely, in the first two back vertebrae, the prespinal laminae that ran alongside the front of the neural spines bulged outwards near the bottom.

At the sides of the front tail vertebrae, the transverse processes (on which the diapophyses were located) were high and thin, like Futalognkosaurus and Mendozasaurus but unlike Dreadnoughtus and other titanosaurs.

Unlike the Diplodocidae, where the transverse processes of the front tail vertebrae formed wing-like shapes, they were wider at the bottom in Patagotitan.

These expanded transverse processes would have resulted in greater attachment areas for the caudofemoralis longus, ilio-ischiocaudalis, and spinalis muscles, and therefore a more muscular tail.

Also like Futalognkosaurus, Mendozasaurus, and Drusilasaura, well-developed spinodiapophyseal laminae (SDLs) ran from the neural spines down the tranverse processes to the diapophyses.

[1] Patagotitan had a robust, expanded scapula, with the blade being only 4.15 times as long as it was wide at its narrowest point; except for Drusilasaura, Isisaurus, Neuquensaurus, and Saltasaurus, this figure was usually considerably higher among the Macronaria.

In addition to a projection at the bottom of the scapula just behind the shoulder joint (glenoid), which was common among titanosaurs, there was a second one further back at the beginning of the blade, only seen otherwise in Alamosaurus, Dreadnoughtus, Drusilasaura, Elaltitan, and Sauroposeidon.

The articulating surface of the coracoid with the scapula was 80% as long front-to-back as it was tall, an uncommon characteristic shared with Giraffatitan, Ruyangosaurus, and Tapuiasaurus.

There was a ridge running diagonally from the outer bottom corner of each sternal plate that was oriented similarly to Dreadnoughtus and Opisthocoelicaudia.

In a depression (fossa) on the front upper portion of the humerus, there was a pair of scars above each other, with the top one being wider than tall and the bottom one being taller than wide; this was where the coracobrachialis muscle inserted.

Like the humerus, the femur of Patagotitan was robust, being on average 23% as broad as it was long, similar to Rocasaurus but somewhat less than Diamantinasaurus, Opisthocoelicaudia, and Saltasaurus.

Uniquely among sauropods, at the outer edge of the bottom of the femur, there were a series of coarse ridges right above the articulation known as the lateral condyle; this was where the flexor digitorum longus muscle inserted.

[2] In 2017, a phylogenetic analysis by Carballido and colleagues placed Patagotitan as the sister group to its closest relative, Argentinosaurus; the two were united by the presence of long neural spines in the front back vertebrae.

Lastly, unlike the results of prior analyses,[18][20][21] the analysis of Carballido and colleagues placed Lognkosauria outside the Lithostrotia, as separate groups within the Eutitanosauria and in turn the Titanosauria.

[1] Wintonotitan Andesaurus Ruyangosaurus Malarguesaurus Epachthosaurus Dreadnoughtus Malawisaurus Baurutitan Nemegtosaurus Trigonosaurus Alamosaurus Opisthocoelicaudia Saltasaurus Neuquensaurus Rapetosaurus Isisaurus Tapuiasaurus Rinconsaurus Muyelensaurus Aeolosaurus Overosaurus Bonitasaura Notocolossus Mendozasaurus Futalognkosaurus Quetecsaurus Puertasaurus Drusilasaura Argentinosaurus Patagotitan Historically, the interrelationships of titanosaurs have been highly unstable and varied between different analyses.

[28] Also in 2021, Pablo Gallina and colleagues recovered a similar arrangement, except with Ninjatitan possibly being closer to Patagotitan and Argentinosaurus than Puertasaurus.

A new genus of cypress similar to the living Fitzroya and Pilgerodendron, Austrocupressinoxylon, was named from the fossilized wood in 2019; these trees were estimated as having been 15.3 metres (50 ft) tall.

[34] The fine-grained sandstone and tuff-bearing siltstone sediments of the Cerro Castaño Member, from which the specimens of Patagotitan were uncovered, are indicative of low-energy water flow on a floodplain, likely related to sporadic sheet flooding.

[1][35] Animals from other localities in the Cerro Castaño Member include the theropod dinosaurs Tyrannotitan[36] and Genyodectes,[37] the peirosaurid crocodyliform Barcinosuchus,[38] and the cryptodire turtle Chubutemys.