Diplomystus is an extinct genus of double-armored herring from the Eocene of Wyoming. It was described by Edward Drinker Cope in 1877. The type species of this genus is Diplomystus dentatus.
Diplomystus possessed a laterally compressed body and a deep belly. The dorsal fin was halfway down the body. It looked a bit like a herring in appearance. It had a deeply forked caudal fin, much like a herring.
Some specimens are found with another clupeomorph called Knightia. Diplomystus was possibly both a freshwater and saltwater fish. Some fossils of D. dentatus had prey inside their body. The bigger forms were active predators. The smaller forms were also predators, but they only ate smaller organisms.