The coho salmon (Oncorhynchus kisutch; Karuk: achvuun) is a species of anadromous fish in the salmon family and one of the six Pacific salmon species. Coho salmon are also known as silver salmon (or "silvers") and is often sold as medium red salmon. The scientific species name is based on the Russian common name kizhuch (кижуч).

During their ocean phase, coho salmon have silver sides and dark-blue backs with spots on their back and upper tail lobe. During their spawning phase, their jaws and teeth become hooked. After entering fresh water, they develop bright-red sides, bluish-green heads and backs, dark bellies and dark spots on their backs. Sexually maturing fish develop a light-pink or rose shading along the belly, and the males may show a slight arching of the back. Mature adults have a pronounced red skin color with darker backs and spots, with females having darker shades than males.

Coho salmon average 20 to 28 inches (50.8 to 71 cm) and 7 to 11 pounds (3.2 to 5.0 kg), occasionally reaching up to 36 pounds (16 kg). Size can vary depending on age and geographic location. Males tend to be slightly larger than females. Mature adults also develop a large kype (hooked beak) which is used to attract a mate during spawning, with males having a more pronounced kype than females. The coho salmon's lower jaw can be distinguished by a light shade at its superior edge.

Once the mature coho has reached three or four years old, it swims up freshwater rivers and streams to spawn. Coho will swim up streams as far as they are physically able, usually reaching areas of water as little as 5 centimetres (2.0 in) deep. Females seek out beds of gravel at the head of a riffle, where they turn on their sides to dig a nest with movements of their tail, creating round or oval depressions roughly the same lengths and width as the fish. This process is repeated for up to seven nests, each called a redd. Females become extremely aggressive with each other over nesting sites, and with males until these are dug. Males then fight for the right to mate. Once a female has chosen a mate, usually the largest male, she lays her eggs onto the redd, while he simultaneously releases milt (sperm) onto the eggs. Unchosen males also sneak in to release milt at this time. Once all eggs are laid, she covers them with rocks and pebbles using her tail. The adults then begin semelparity, whereby they stop eating and deteriorate to death.

The eggs hatch in the late winter or early spring after six to seven weeks in the redd. Once hatched, they remain mostly immobile in the redd during the alevin life stage, which also lasts for six to seven weeks. Alevin no longer have the protective egg shell, or chorion, and rely on their yolk sacs for nourishment during growth. The alevin life stage is very sensitive to aquatic and sedimental contaminants. When the yolk sac is completely resorbed, the alevin leaves the redd. Young coho spend one to two years in their freshwater natal streams, often spending the first winter in off-channel sloughs, before transforming to the smolt stage. Smolts are generally 100–150 mm (3.9–5.9 in) and as their parr marks fade and the adult's characteristic silver scales start to dominate. Smolts migrate to the ocean from late March through July. Some fish leave fresh water in the spring, spend summer in brackish estuarine ponds, and then return to fresh water in the fall. Coho salmon live in salt water for one to three years before returning to spawn. Some precocious males, known as "jacks", return as two-year-old spawners. Spawning males develop kypes, which are strongly hooked snouts and large teeth.

The traditional range of the coho salmon runs along both sides of the North Pacific Ocean, from Hokkaidō, Japan and eastern Russia, around the Bering Sea to mainland Alaska, and south to Monterey Bay, California. Coho salmon have also been introduced in all the Great Lakes, as well as many landlocked reservoirs throughout the United States. Coho salmon were first introduced in Lake Erie in the 1920s to control the lake's alewife population. Large-scale stocking began in 1966, when 660,000 fingerlings were introduced in Lake Michigan. Wisconsin began a program of introduction of 500,000 fry a year, mainly on the coastline of Lake Michigan. By 1970, the species was present in all of the Great Lakes. Ontario and Minnesota adopted stocking programs in 1969, but later abandoned them. Colorado began stocking coho salmons in the early 1900s, although only limited reproduction is recorded in the Colorado River. Another stocking was done in New Hampshire in the late 1960s, which dispersed to Maine and Massachusetts and were recorded to engage in sporadic reproduction.

Natural reproduction in introduced areas is generally low. Limited self-sustaining populations are recorded in Lake Superior and Michigan, but the salmon does not reproduce naturally in the Wisconsin tributaries of Lake Michigan and relies on stocking to maintain a large population. The species is no longer stocked in Lake Erie, although stray individuals from elsewhere in the Great Lakes are caught on occasion. Efforts to establish the species in Connecticut began in the 1800s, but were discontinued when the species failed to establish a stable population. Introductions in Connecticut and Delaware also failed to found permanent populations.

Over twenty specimens were caught in waters surrounding Denmark and Norway in 2017. Their source is currently unknown, but the salmon species is farmed at several locations in Europe, making it probable that the animal has slipped the net at such a farm.