The king snake eel (Ophichthus rex) is an eel in the family Ophichthidae (worm/snake eels). It was described by James Erwin Böhlke and John H. Caruso in 1980. It is a marine, tropical eel which is known from Florida to Texas, USA, in the northern Gulf of Mexico in the western Atlantic Ocean. It dwells at a depth range of 15 to 365 meters (49 to 1,198 ft), and inhabits offshore waters. King snake eels can reach a total length of 211 centimeters (83 in); the maximum recorded weight is 23.6 kilograms (52 lb). caught by Patrick Lemire on the Texsun II out of Galveston, Texas, in 1997.

The King snake eel is often caught near oil platforms by anglers. The King snake eel is often caught near oil platforms by anglers. This eel is sometimes also referred to as "giant snake eel", mud eel (in reference to their preferred burrowing substrate), slime eel, worm eel, and keoghfish.

The name rex is from the Latin word meaning king. It is believed to be named this for the large size that the species is known for.