Loncosaurus (meaning uncertain; either Araucanian "chief" or Greek "lance" "lizard"[1]) is an extinct genus of ornithopod dinosaur from the Upper Cretaceous of Provincia de Santa Cruz, Argentina.

The type (and only known) species is Loncosaurus argentinus, described by the famous Argentinian paleontologist Florentino Ameghino, but is considered a dubious name.

Ameghino named this dinosaur in 1899[3][4][5][6][7][8][9][10][11] from a proximal femur (MACN-1629) and tooth found near Rio Sehuen, Santa Cruz, in the Cardiel Formation[10] (Upper Cretaceous).

Either way, he thought the remains belonged to a "megalosaurid" dinosaur, a carnivore, which Friedrich von Huene agreed with.

[12] Upon further review, von Zittel assigned it to the Coeluridae,[13] recognized today as a "wastebasket taxon" for small carnivorous dinosaurs.