The type species, L. sauropoides, was formally described by José Fernando Bonaparte in 1999 in honor of Don Lessem, a writer of popular science books.

It was found in the Los Colorados Formation of the Ischigualasto-Villa Unión Basin in La Rioja Province, Argentina.

[1] It was discovered in strata dating to the Norian stage, around 210 million years ago.

[4][5][6][7] A cladogram after Pol, Garrido & Cerda, 2011,[8] illustrates a possible placing of Lessemsaurus and Antetonitrus in Sauropodomorpha: Saturnalia Pantydraco Thecodontosaurus Efraasia Ruehleia Plateosaurus Riojasaurus Massospondylus Coloradisaurus Lufengosaurus Gyposaurus Yunnanosaurus Anchisaurus Aardonyx Leonerasaurus Melanorosaurus Antetonitrus Lessemsaurus Kotasaurus Vulcanodon Eusauropoda In 2018, Apaldetti et al. recovered it as part of a clade they named Lessemsauridae, after Lessemsaurus.

Their cladogram is reproduced below:[9] Mussaurus Aardonyx Sefapanosaurus Camelotia Melanorosaurus Antetonitrus Ingentia Lessemsaurus Blikanasaurus Gongxianosaurus Pulanesaura Gravisauria The following cladogram shows the position of Lessemsaurus outside of Sauropoda, according to Oliver W. M. Rauhut and colleagues, 2020:[10] Eucnemesaurus Riojasaurus Sarahsaurus Yunnanosaurus Jingshanosaurus Seitaad Coloradisaurus Glacialisaurus Lufengosaurus Massospondylus Adeopapposaurus Leyesaurus Xingxiulong Anchisaurus Leonerasaurus Mussaurus Aardonyx Sefapanosaurus Meroktenos Melanorosaurus Camelotia Lessemsaurus Antetonitrus Ingentia Blikanasaurus Pulanesaura Gongxianosaurus Schleitheimia Isanosaurus Tazoudasaurus Sauropoda