Although snakes are descended from limbed squamates (’lizards’), all known snakes lack well developed legs and their nearest lizard relatives have yet to be identified(1-4). Here we provide compelling evidence that the Cretaceous squamate Pachyrhachis problematicus, previously interpreted as a varanoid lizard(5-7), is actually a primitive snake with a well developed pelvis and hindlimbs. Pachyrhachis is the sister-taxon of all other snakes. The skull exhibits most derived features of modern snakes, and the body is slender and elongated. But unlike other snakes, Pachyrhachis retains a well developed sacrum, pelvis and hindlimb (femur, tibia, fibula, tarsals). Pachyrhachis was marine, and provides additional support for mosasauroid-snake affinities.