The longfin smelt, Spirinchus thaleichthys, is a smelt that is found in several estuaries and lakes along the northern Pacific coast of North America.

Its most distinctive characteristic is the long pectoral fins that reach nearly to the base of the pelvic fins, and thus inspire the common name. The sides are silver, with the back ranging from an olive to a pinkish shade. The upper jaw is long, reaching nearly to the posterior edge of the eye, and the lower jaw projects slightly in front of the upper jaw. Both jaws have a set of very small teeth. The lateral line is incomplete, reaching back only as far as the dorsal fin. Size is limited to about 20 cm.