Pachyrhachis (from Greek: παχύς pakhús, 'thick' and Greek: ῥάχῐς rhákhis, 'spine') is an extinct genus of snake with well developed hind legs known from fossils discovered in Ein Yabrud, near Ramallah, in the central West Bank.

It is a relatively small snake, measuring more than 1.5 metres (4.9 ft) long at maximum.

[1] Pachyrhachis appears to have been an ancient marine snake; the fossils occur in a marine limestone deposit, and the thickened bone of the ribs and vertebrae would have functioned as ballast to decrease the buoyancy of the animal, allowing it to dive beneath the ancient Cretaceous seas that it once inhabited.

The position of Pachyrhachis within snakes has been debated (e.g. Lee and Scanlon 2002; Rieppel et al. 2003).

Pachyrhachis is among the oldest known snakes and retains well-developed hind limbs, suggesting it represented a transitional form linking snakes to marine lizards (Lee and Scanlon 2002), though other studies place Pachyrhachis within the modern snake radiation as stem-Macrostomata (Zaher, 1998; Zaher & Rieppel, 1999).