Oncorhynchus is a genus of fish in the family Salmonidae; it contains the Pacific salmons and Pacific trouts. The name of the genus is derived from the Greek onkos ("hook") and rynchos ("nose"), in reference to the hooked jaws of males in the mating season (the "kype").

Salmon and trout with ranges generally in waters draining to the Pacific Ocean are members of the genus. Their range extends from Beringia southwards, roughly to Japan in the west and Mexico to the east. In North America, some subspecies of O. clarki are native to the landlocked Great Basin, while others are native to the Rio Grande and western tributaries of the Mississippi River Basin which drain to the Gulf of Mexico, rather than to the Pacific.

Unlike many trout species of the mainly European genus Salmo, many Oncorhynchus are anadromous (migratory) and semelparous (die after spawning). Some species of char (Salvelinus genus) are native to Pacific waters and are also referred to as trout.

Several late Miocene (~7 m.y.a.) trout-like fossils appear in Idaho, in the Clarkia Lake beds appear to be Oncorhynchus—the current genus for Pacific salmon and some trout. The presence of these species so far inland established that Oncorhynchus was not only present in the Pacific drainages before the beginning of the Pliocene (~5–6 m.y.a.), but also that rainbow and cutthroat trout, and Pacific salmon lineages had diverged before the beginning of the Pliocene. Consequently, the split between Oncorhynchus and Salmo (Atlantic salmon) must have occurred well before the Pliocene. Suggestions have gone back as far as the early Miocene (~20 m.y.a.). One fossil species assigned to this genus is Oncorhynchus rastrosus, the Sabertooth salmon (sometimes called Smilodonichthys), a 9-foot-(3 meter-)long species known from Late Miocene to Pleistocene fossil.