Najash rionegrina is an extinct basal snake from the Late Cretaceous Candelerous Formation of Patagonia. Like a number of other Cretaceous and living snakes it retained hindlimbs, but Najash is unusual in having well-developed legs that extend outside the rib cage, and a pelvis connected to the spine. Fossils of Najash were found in the terrestrial Candeleros Formation, in Rio Negro Province, Argentina, and date to roughly 90 million years ago. The skull and spine of Najash both show adaptations for a subterranean existence, consistent with the hypothesis that the long bodies and reduced limbs of snakes are an adaptation for burrowing.

This burrowing creature had not lost its sacrum, the pelvic bone composed of several fused vertebrae, nor its pelvic girdle which are absent in modern snakes, and in all other known fossil snakes as well. Several phylogenetic analysis place Najash as either the most primitive known snake or near the base of the snake radiation, but outside of all living snakes.

This discovery does not support the hypothesis, first offered by the nineteenth-century paleontologist Edward Drinker Cope, that snakes share a common marine ancestry with Mosasaurus. The marine origin hypothesis received new impetus with the discovery in the 1990s of basal snakes with vestigial limbs in marine sediments in Lebanon.

The generic name comes from the biblical legged snake of Genesis, Nahash, who tempted Adam and Eve to eat from a forbidden fruit tree.

Art impression of Najash rionegrina