Gammarus is an amphipod crustacean genus in the family Gammaridae.

It contains more than 200 described species, making it one of the most species-rich genera of crustaceans.

[2] Different species have different optimal conditions, particularly in terms of salinity, and different tolerances; Gammarus pulex, for instance, is a purely freshwater species, while Gammarus locusta is estuarine, only living where the salinity is greater than 25‰.

[3] Species of Gammarus are the typical "scuds" of North America and range widely throughout the Holarctic.

A considerable number are also found southwards into the Northern Hemisphere tropics, particularly in Southeast Asia.