P. rex may refer to:
Papilio rex, the regal swallowtail, a butterfly species found in Africa Pedicularis rex, a plant related to lousewort Percina rex, the Roanoke logperch, a small freshwater fish species found in the Roanoke and Chowan drainages Potamotrygon rex, the great river stingray, a freshwater stingray species native to the Rio Tocantins in Brazil Pseudoeurycea rex, a salamander species found mostly in Guatemala and Mexico
See also Rex (disambiguation)