[2][3] P. rex, along with some other Potamotrygon species form a sub-species group known as "black stingrays" known by their similar features and their locality to rivers draining the Central Brazilian Shield.

Its appearance is dark brown to blackish with distinct clusters of yellow-orange spots around similarly colored ocelli on its dorsal side.

Distinguishing features include broad pelvic-fin apices, lack of labial grooves, irregular double row of dorsal tail thorns, and having two angular cartilages associated with each hyomandibula.

[4] Potamotrygon rex is a neotropical freshwater stingray in the family Potamotrygonidae from the middle and upper Rio Tocantins, Brazil.

Potamotrygon rex, P. henlei, P. leopoldi and another new Potamotrygon species from the Rio Tapajós form a species-group (the "black stingrays") occurring in rivers draining the central Brazilian shield, characterized by their black to dark brown but highly ornate dorsal color, wide pelvic fins with broadly convex apices, among other features.

However, all species of freshwater stingray belonging to the genus Potamotrygon are listed in CITES Appendix III, which combats the illegal international trade of such animals.