Lessemsaurus is an extinct genus of sauropodiform dinosaur that lived in the Norian-aged (Upper Triassic) Los Colorados Formation of what is now Argentina.
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.
It is part of the La Esquina faunal assemblage, dating to the mid-Norian stage, around 213 million years ago. It is estimated to have reached long and weighed over , possibly up to , in maximum body mass.
A cladogram after Pol, Garrido & Cerda, 2011, illustrates a possible placing of Lessemsaurus and Antetonitrus in Sauropodomorpha:
In 2018, Apaldetti et al. recovered it as part of a clade they named Lessemsauridae, after Lessemsaurus. Their cladogram is reproduced below:
The following cladogram shows the position of Lessemsaurus outside of Sauropoda, according to Oliver W. M. Rauhut and colleagues, 2020: