The redside shiner (Richardsonius balteatus) is a species of cyprinid fish found in the Western United States and British Columbia.

The species has been introduced into northwestern Colorado, southwestern Wyoming, the upper Missouri River drainage in Montana and in reservoirs outside its native distribution in southern Utah.

Some of these introductions are from illegal bait bucket transfers, likely done by well-meaning anglers to increase forage for sport fish, but not an ecologically sound decision in most cases.

Subspecies R. b. hydrophlox is distributed in Utah's Bonneville Basin and waters in the Snake River Plain above Shoshone Falls as well as a disjunct population in the Malheur region of southeast Oregon.

It is broadly accepted that geological processes (river capture and lake spillover events such as the Bonneville flood) and changing climate (survival in and expansion from Pleistocene refugia) have contributed to this species current geographic distribution as described by Hubbs and Miller (1948),[3] Minckley et al (1981),[6] McPhail and Lindsey (1981).

Additional red coloration is exhibited on the lower half of the anterior end of the body during spring spawning, especially in males.