Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton discovered in the Ischigualasto Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina.

[3] The genus is based on a specimen given the designation PVL 2577, which consists of a partial skull including a fragmentary right maxilla with teeth, and incomplete right mandibular ramus (lower jaw), six incomplete cervical vertebrae, seven incomplete dorsal vertebrae, molds of five sacral vertebrae, a rib and several rib fragments, a fragmentary right scapula, a coracoid, molds of a fragmentary ilium, ischium and pubic bone, an impression of three metacarpals, the complete femora, the right tibia, the right fibula, with an articulated astragalus and calcaneum, a tarsal element with a metatarsal, metatarsals III and IV, three phalanges from the third toe and five phalanges (including the ungual) from the fourth toe, and an indeterminate long bone fragment.

Based on the known fossil elements from a partial skeleton, Pisanosaurus was a small, lightly built dinosauriform, reaching 1–1.3 m (3.3–4.3 ft) in length and 2 kg (4.4 lb) in body mass.

[1] Pisanosaurus has traditionally been classified as very basal within Ornithischia; the postcrania seem to lack any good ornithischian synapomorphy and it was even suggested by Paul Sereno in 1991 that the fossil is a chimera.

[1] A phylogenetic analysis informally conducted by Agnolin (2015) recovered Pisanosaurus as a possible non-dinosaurian member of Dinosauriformes related to the silesaurids.

[17] In 2017, two studies independently came to the conclusion that Pisanosaurus was a silesaurid: one was an expansive redescription by Agnolin and Rozadilla,[18] and the other was a re-analyzed Ornithoscelida matrix by Baron, Norman, & Barrett.

[11] Pisanosaurus was also found as a silesaurid in a 2018 paper which combined the descriptive work of Agnolin and Rozadilla (2017) with the phylogenetic matrix of Baron, Norman, & Barrett (2017).

[20][21] The fossils of Pisanosaurus were discovered in the "Agua de las Catas" locality at the Ischigualasto Formation in La Rioja, Argentina.

The Ischigualasto Formation was a volcanically active floodplain covered by forests, with a warm and humid climate,[22] though subject to seasonal variations including strong rainfalls.

[25] Sereno (1993) noted that Pisanosaurus was found in "close association" with therapsids, rauisuchians, archosaurs, Saurosuchus and the dinosaurs Herrerasaurus and Eoraptor, all of whom lived in its paleoenvironment.