Omega 3 fatty acids (also called3 fatty acids or n3 fatty acids) are fats commonly found in marine and plant oils. They are polyunsaturated fatty acids with a double bond (C=C) starting after the third carbon atom from the end of the carbon chain. The fatty acids have two end the acid (COOH) end and the methyl (CH₃) end. The location of the first double bond is counted from the methyl end, which is also known as the omega () end or the n end.

The health effects of n-3 fatty acids supplementation are controversial. They are considered essential fatty acids, meaning that they cannot be synthesized by the human body but are vital for normal metabolism. Though mammals cannot synthesize 3 fatty acids, they have a limited ability to form the long-chain 3 fatty acids including eicosapentaenoic acid (EPA, 20 carbons and 5 double bonds), docosahexaenoic acid (DHA, 22 carbons and 6 double bonds) and +linolenic acid (ALA, 18 carbons and 3 double bonds).

Common sources of 3 fatty acids include fish oils, algal oil, squid oil, and some plant oils such as echium oil and flaxseed oil.