Eicosapentaenoic acid is an Ω-3 polyunsaturated fatty acid with 20 carbons and five double bonds, the first double bond is located at the third carbon from the omega end. It also has the trivial name Timnodonic acid.

It is used to treat asthma, cancer, arthritis, Lupus, blood clotting, gingivitis (gum disease), high cholesterol, hypertension (high blood pressure), colitis (inflammatory bowel disease), Crohn's disease, and psoriasis. EPA is also used as an antiinflammatory (help with pain and swelling), to stimulate the immune system, and for cardiovascular health, to help prevent heart disease and stroke. It may also be used to prevent Alzheimer's disease.

Eicosapentaenoic acid and its metabolites act in the body largely by their interactions with the metabolites of arachidonic acid.

It is obtained in the human diet by eating oily fish or fish oil - cod liver, herring, mackerel, salmon, and sardine. It is also found in human breast milk.

It is available from some non-animal sources-spirulina and microalgae. Microalgae are being developed as a commercial source. Eicosapentaenoic acid is not usually found in higher plants, but it has been reported in trace amounts in purslane.

The US National Institute of Health's MedlinePlus lists a large number of conditions in which Eicosapentaenoic acid (alone or in concert with other Ω-3 sources) is known or thought to be effective. Most of these involve its ability to lower inflammation.

Among Ω-3 fatty acids, in particular Eicosapentaenoic acid is thought to possess beneficial potential in mental conditions, such as schizophrenia. Several studies report an additional reduction in scores on symptom scales used to assess the severity of symptoms, when additional Eicosapentaenoic acid is taken.

Marine fatty acids, particularly the long-chain Eicosapentaenoic acid and Docosahexaenoic acid, have been consistently shown to inhibit the proliferation of breast and prostate cancer cell lines in vitro and to reduce the risk and progression of these tumors in animal experiments. However, whether a high consumption of marine fatty acids can reduce the risk of these cancers or other hormone-dependent cancers in human populations is unclear.

Long-chain Ω-3 fatty acids, Eicosapentaenoic acid and Docosahexaenoic acid, are associated with decreased triglyceride levels in hypertriglyceridemic patients and decreased risk of developing coronary heart disease (CHD).

Vegetarian, especially vegan, diets are relatively low in alpha-linolenic acid compared with Linoleic acid and provide little, if any, Eicosapentaenoic acid and Docosahexaenoic acid. Clinical studies suggest that tissue levels of long-chain n-3 fatty acids are depressed in vegetarians, particularly in vegans.

α-Parinaric acid also found in the seed fat from Impatiens balsamina and mackrel oil.

Eicosapentaenoic acid, is also effective in preventing loss of skeletal muscle in cancer patients.