It is known from well-preserved fossils found in the Bridger and Green River Formations of Wyoming, United States.

[2] Although similar in appearance to extant monitor lizards, Saniwa had many primitive traits, including teeth on its palate, a jugal bone beneath the eye that extended farther forward, and a suture between the frontal and parietal bones that was straight rather than curved.

[5] In 1870, American geologist Ferdinand Vandeveer Hayden found the first fossils of Saniwa near the town of Granger, Wyoming, and gave them to paleontologist Joseph Leidy.

Hayden suggested the name Saniwa to Leidy because it was "used by one of the Indian tribes of the Upper Missouri for a rock-lizard.

He also named a second species, Saniwa [sic] major, on the basis of a broken humerus and some isolated dorsal vertebrae.

[7] In 1918, Baron G. J. de Fejérváry suggested that S. major was not a species of lizard, noting that the humerus was "undoubtedly" nonreptilian.

Soon after Leidy named Saniwa, American paleontologist Othniel Charles Marsh erected the genus Thinosaurus in 1872 for several species of extinct lizards in the western United States.

In 1899, Argentine paleontologist Florentino Ameghino named another species of Saniwa, S. australis, from lower Miocene rocks in Argentina.

Unlike the Argentine fossils, the Belgian remains represent a definite occurrence of Saniwa outside North America.

[2] Since its first description, Saniwa has been recognized as a close relative of living monitor lizards in the genus Varanus.