The rainbow trout (Oncorhynchus mykiss) is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America as well as much of the central, western, eastern, and especially the northern portions of the United States. The ocean going (anadromous) form (including those returning for spawning) are known as steelhead, or ocean trout (Australia). The species has been introduced for food or sport to at least 45 countries, and every continent except Antarctica.

Rainbow trout are unusual in that there are two forms which sometimes share the same habitat. The anadromous form called "steelhead" migrate to the ocean, though they must return to fresh water to reproduce. The freshwater form is called "rainbow trout"[3], based on the broad red band along their sides. Steelhead are exactly the same species as rainbow trout. However, the difference is anadromy. After going to sea, their color changes, including loss of the red band. They stay at sea for 1-4 years, and return to fresh water to spawn. Rainbows stay in fresh water their whole lives. It is a common misconception that "redband trout" and "rainbow trout" are the same fish. While closely related, redbands are a different subspecies, somewhat intermediate between rainbow and cutthroat trout (Oncorhynchus clarki). Redbands have larger spots and darker colors than rainbows and sometimes have cutthroat-like slash marks, but not the cutthroat's longer maxilla or hyoid (basibranchial) teeth. Redbands include anadromous steelhead forms such as the "redsides" of the Columbia River system above the Columbia River Gorge and similar fish from the Fraser River system above Fraser Canyon and the most inland parts of the Sacramento River, Klamath River and others flowing into the Pacific. Redbands are also found in Endorheic basins partly or wholly isolated since the Pleistocene including Goose Lake (Oregon-California) which only intermittently overflows into the Pit River; the Kern River which has lost its connection to the San Joaquin River; and other basins that have lost connections to the Columbia-Snake, Sacramento or Klamath systems since the Pleistocene. However basins isolated much longer tend to have endemic varieties of Cutthroat trout. Steelhead also tend to be more silvery than the freshwater rainbow form. Lake and stream dwellers tend to be lighter as well. River dwelling rainbow trout are generally darker and more brilliant. In all habitats, rainbow's backs varies from blue or green to a yellow-green or brown. They always have dark spots on their heads, backs, bellies and across their dorsal fins and caudal fins, a famed attribute of true trout. Rainbows also have a red or pink streak that runs from the gill cover to the caudal fin, inspiring their name. Steelhead usually lack the pink stripe and have chrome-colored sides. Unlike other Pacific Salmon, rainbow trout and steelhead do not necessarily die after spawning (they may spawn as many as four times). All Pacific salmon, including male steelhead, undergo changes when spawning or migrating to spawn. They change color, head and mouth shape. Steelhead populations and rainbow trout populations can and do interbreed at different levels. It is possible for the offspring of rainbow trout to become anadromous and for the offspring of steelhead to develop a resident life history.