Priocharax rex is a minute species of freshwater ray-finned fish belonging to the family Acestrorhamphidae, the American characins. This fish is considered to be found exclusively in the middle part of the Putumayo River basin in Colombia.
This species reaches a length of .
The fish's name means "king" in Latin, referring to its size as the largest fish of the genus Priocharax.
Priocharax rex is distinguishable by a round flap of skin between contralateral pelvic fins.