Grunion are two fish species of the genus Leuresthes: the California grunion, L. tenuis, and the Gulf grunion, L. sardinas. They are sardine-sized teleost fishes of the New World silverside family Atherinopsidae, found only off the coast of California, USA, and Baja California, Mexico, where the species are found on both the Pacific Ocean and Gulf of California coasts. Many people enjoy catching grunion at events called "grunion runs."

Grunion are known for their unusual mating ritual wherein at very high tides, the females come up on to sandy beaches where they dig their tails into the sand to lay their eggs. The male then wraps himself around the female to deposit his sperm, and for the next 10 days the grunion eggs remain hidden in the sand. At the next set of high tides, the eggs hatch and the young grunion are washed out to sea.

A related species, the false grunion (Colpichthys regis) lives in the Gulf of California. Although the fish looks and acts similarly, it does not have the same breeding method.