Grunion are two fish species of the genus Leuresthes: the California grunion, L. tenuis, and the Gulf grunion, L. sardina. 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 California Grunion are found on the Pacific Ocean coast, and the Gulf Grunion within the Gulf of California. Many people enjoy watching "grunion runs."

Grunion are known for their unusual mating ritual. At semilunar high tides, they ride waves up onto sandy beaches where females dig their tails into the sand to lay their eggs. The males then wrap around the female to provide his sperm. For the entire period of incubation, grunion eggs remain hidden in the sand. At the next set of high tides, about 10 or 12 days later, the eggs hatch rapidly when washed out to sea, releasing the larvae into the water.

A related New World Silverside, the false grunion (Colpichthys regis) lives in the Gulf of California. This fish looks similar, and also spawns in the intertidal zone.

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