Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 0.05%. These environments differ from marine conditions in many ways, the most obvious being the difference in levels of salinity. To survive fresh water, the fish need a range of physiological adaptations.
41.24% of all known species of fish are found in fresh water. This is primarily due to the rapid speciation that the scattered habitats make possible. When dealing with ponds and lakes, one might use the same basic models of speciation as when studying island biogeography.
Freshwater fish differ physiologically from salt water fish in several respects. Their gills must be able to diffuse dissolved gasses while keeping the salts in the body fluids inside. Their scales reduce water diffusion through the skin: freshwater fish that have lost too many scales will die. They also have well developed kidneys to reclaim salts from body fluids before excretion.
Many species of fish do reproduce in freshwater, but spend most of their adult lives in the sea. These are known as anadromous fish, and include, for instance, salmon, trout, sea lamprey  and three-spined stickleback. Some other kinds of fish are, on the contrary, born in salt water, but live most of or parts of their adult lives in fresh water; for instance the eels. These are known as catadromous fish.
Species migrating between marine and fresh waters need adaptations for both environments; when in salt water they need to keep the bodily salt concentration on a level lower than the surroundings, and vice versa. Many species solve this problem by associating different habitats with different stages of life. Both eels, anadromous salmoniform fish and the sea lamprey have different tolerances in salinity in different stages of their lives.
Among fishers in the United States, freshwater fish species are usually classified by the water temperature in which they survive. The water temperature affects the amount of oxygen available as cold water contains more oxygen than warm water. 
Coldwater fish species survive in the coldest temperatures, preferring a water temperature of 50 to 60 °F (10–16 °C). In North America, air temperatures that result in sufficiently cold water temperatures are found in the northern United States, Canada, and in the southern United States at high elevation. Common coldwater fish include brook trout, rainbow trout, and brown trout.
Warmwater fish species can survive in a wide range of conditions, preferring a water temperature around 80 °F (27 °C). Warmwater fish can survive cold winter temperatures in northern climates, but thrive in warmer water. Common warmwater fish include catfish, largemouth bass, bluegill, crappies, and many other species from the Centrarchidae family.
Coolwater fish species prefer water temperature between the coldwater and warmwater species, around 60 to 80 °F (16–27 °C). They are found throughout North America except for the southern portions of the United States. Common coolwater species include muskellunge, northern pike, walleye, and yellow perch
About four in ten North American freshwater fish are endangered, according to a pan-North American study, the main cause being human pollution. The number of fish species and subspecies to become endangered has risen from 40 to 61, since 1989. 
- Freshwater fish in the Philippines
- Freshwater fish of Australia
- Freshwater fish of Barbados
- Freshwater fish of India
- Freshwater fish of Ireland
- Freshwater fish of the United States: Idaho, Maryland, Oklahoma, Oregon, Washington, West Virginia, plus a List of official and unofficial fishes by state
- Lake ecosystems
- List of common fish names
- List of fishes in Bangladesh
- List of fishes of Great Britain
- List of freshwater fishes of Greece
- List of freshwater fishes of Korea
- River ecosystems
- Saltwater fish
- Silva, S., Araújo, M. J., Bao, M., Mucientes, G., & Cobo, F. (2014). The haematophagous feeding stage of anadromous populations of sea lamprey Petromyzon marinus: low host selectivity and wide range of habitats. Hydrobiologia, 734(1), 187-199.
- "Freshwater Fish Species". Retrieved 2016-10-07.
- "Freshwater fish in North America endangered: study". Archived from the original on 2008-09-13. Retrieved 2008-09-11.
- Borgstrøm, Reidar & Hansen, Lars Petter (red): Fisk i ferskvann - et samspill mellom bestander, miljø og forvaltning, Landbruksforlaget 2000
- Jonsson, Bror: «Fiskene» i Norges dyr - Fiskene 1, Cappelen 1992
- Olden, J. D., Kennard, M. J., Leprieur, F., Tedesco, P. A., Winemiller, K. O., & García‐Berthou, E. (2010). "Conservation biogeography of freshwater fishes: recent progress and future challenges". Diversity and Distributions, 16 (3): 496–513. doi: 10.1111/j.1472-4642.2010.00655.x