Saltwater fish, also called marine fish or sea fish, are fish that live in seawater. Saltwater fish can swim and live alone or in a large group called a school.

Saltwater fish are very commonly kept in aquariums for entertainment. Many saltwater fish are also caught to be eaten, or grown in aquaculture. However, many fish species have been overfished and are otherwise threatened by marine pollution or ecological changes.

Fishes that live in the ocean can be carnivores, herbivores, or omnivores. Herbivores in the ocean eat things such as algae and flowering seagrasses. Many herbivores' diets consist of primarily algae. Most saltwater fish will eat both macroalgae and microalgae. Many fish eat red, green, brown, and blue algae, but some fish prefer other types. Most saltwater fish that are carnivores will never eat algae under any circumstances. Carnivores' diets consist of shrimp, plankton, or tiny crustaceans.

Saltwater aquariums are a multi-million dollar industry in the United States. About 10 million marine fish are imported into the United States each year for aquarium use. The United States imports more saltwater fish than any other country in the world. There are approximately 2,000 different species of saltwater fish that are imported and used in captivity. In many circumstances, fish used for marine trade are collected using harmful tactics such as cyanide. One way that people are trying to protect the coral reefs is by breeding marine fish in captivity. Captive-bred fish are known to be healthier and likely to live longer. Captive-bred fish are less susceptible to disease because they have not been exposed to the wild and they have not been damaged during the shipment process. Fish that are bred in captivity are already accustomed to aquarium habitats and food.

There are many different components that make up a marine life habitat. Some of them are the temperature of the water, the quality, and quantity of water (flow and depth). Other components that can also contribute to the habitat of saltwater fish are pH level, salt level, and alkalinity level. Levels of nitrates and phosphates are also relevant, particularly when considering conditions for fish in captivity. There are other physical features that contribute to a habitat which are physical materials like rocks, reefs, and sand or the vegetation like the amount of algae, water plants, and saltmarsh. Specific fish live in specific habitats based on what they eat or what cycle of life they are currently at, another thing is the amount of salt that is in the water at that specific location. Some ocean habitats are not technically in the ocean and these are called estuaries, areas when oceans and rivers meet creating a mixture of salt water and freshwater making a different habitat for different types of fish and creatures to live in. The ocean is home to organisms as large as whales and as small as microscopic marine organisms such as phytoplankton. However, the vast majority of ocean life that humans are exposed to is simple saltwater fish. Saltwater fish can live in the deepest depths of the ocean where no sunlight can penetrate, but they can also live on the surface of the water.

Marine fish face many anthropogenic threats. Common human-induced threats include overfishing, pollution, habitat loss and destruction, climate change and invasive species. The aforementioned threats all come with a multitude of negative direct and indirect affects to marine ecosystems. With the human population growing at an exponential rate, these threats are likely to continue to be prevalent in marine ecosystems.

Overfishing is defined as the mass removal of fish from a body of water that results in halting the ability for breeding populations to replenish what has been removed. Fish is one of the most popular foods in the world and consumption has continued to rise with the growing human population and will continue to do so. The value of the global seafood market has seen a 15% increase from 2016 to 2020 and is projected to increase even more by 2023. Although it provides many people with a source of food, the global seafood market is a major threat to the biodiversity of fishes. Bycatch is a direct effect of overfishing and is defined as the unwanted capture of different marine organisms during industrial fishing. This results in many different species of fish dying after they are captured and discarded. Data on bycatch is often unclear and not well recorded but it is estimated that the U.S. alone discards 17–22% of their catch annually. The Mesopredator release hypothesis is one of the indirect effects of overfishing that is also often referred to as "fishing down the food web". This phenomenon means as fisheries deplete large apex predatory species, mid-sized predatory species increase in abundance and assume the role as top predators on the food web. This impacts the food web in marine environments and disrupts the balance of the ecosystem and is likely to cause trophic cascades.

Profitable fish stocks like the bluefin tuna are decreasing in numbers because of high demand. According to the IUCN Red List, the Pacific bluefin tuna, Atlantic bluefin tuna, and southern bluefin tuna are classified as vulnerable, endangered and critically endangered all due to over-exploitation.