The anatomy of Pachyrhachis problematicus, an elongate, limb–reduced squamate from the Upper Cretaceous of Israel, is described and evaluated in detail. Previously considered a snake–like ‘lizard’ of uncertain affinities, it is here shown to be the most primitive snake, and the sister–group to all other snakes. Pachyrhachis exhibits numerous derived characters uniting it with modern snakes (scolecophidians and alethinophidians): e.g. mobile premaxilla–maxilla articulation, braincase enclosed by frontals and parietals, sagittal parietal crest, absence of tympanic recess, single postdentary bone, over 140 presacral vertebrae, and complete loss of shoulder girdle and forelimb. However, it is more primitive than all modern snakes in retaining some strikingly primitive (lizard–like) features: presence of a jugal, squamosal, normal sacral attachment, and well–developed hindlimb composed of femur, tibia, fibula, and tarsals. Pachyrhachis provides additional support for the hypothesis that snakes are most closely related to Cretaceous marine lizards (mosasauroids). Almost all of the derived characters proposed to unite snakes and mosasauroids are highly developed in Pachyrhachis: the mobile mandibular symphysis, intramandibular joint, long and recurved pterygoid teeth, quadrate suspended by the supratemporal, loosely united pelvic elements (ilium, ischium, and pubis), and separate astragalus and calcaneum.