Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6), or 20:4(5,8,11,14). It is structurally related to the saturated arachidic acid found in cupuaçu butter.

Arachidonic acid is a polyunsaturated fatty acid present in the phospholipids (especially phosphatidylethanolamine, phosphatidylcholine, and phosphatidylinositides) of membranes of the body's cells, and is abundant in the brain, muscles, and liver. Skeletal muscle is an especially active site of arachidonic acid retention, accounting for roughly 10-20% of the phospholipid fatty acid content typically.

Arachidonic acid is not one of the essential fatty acids. However, it does become essential if a deficiency in linoleic acid exists or if an inability to convert linoleic acid to arachidonic acid occurs. Some mammals lack the ability or have a very limited capacity to convert linoleic acid to arachidonic acid, making it an essential part of their diets. Since little or no arachidonic acid is found in common plants, such animals are obligate carnivores; the cat is a common example having inability to desaturate essential fatty acids. A commercial source of arachidonic acid has been derived, however, from the fungus Mortierella alpina.