Alpha-Linolenic acid (ALA) is an organic compound found in many common vegetable oils. Systematically, it is named all-cis-9,12,15-octadecatrienoic acid.

Alpha-Linolenic acid is a carboxylic acid with an 18-carbon chain and three cis double bonds. The first double bond is located at the third carbon from the omega end. Thus, Alpha-Linolenic acid is both a polyunsaturated fatty acid, and an Ω-3 fatty acid.

Seed oils are the richest sources of alpha linolenic acid, notably those of rapeseed (canola), soybeans, walnuts, flaxseed (Linseed), perilla, chia and hemp. Alpha linolenic acid is also obtained from the thylakoid membranes of the green leaves of broadleaf plants (the membranes responsible for photosynthesis).

Alpha-Linolenic acid is a member of the group of essential fatty acids called Ω-3 fatty acids, so called because they are an essential dietary requirement for all mammals. Most seeds and seed oils are much richer in the Ω-6 fat linoleic acid. Linoleic acid is also an essential fat, but it, and the other Ω-6 fats, compete with Ω-3s for positions in cell membranes and have very different effects on human health.

Studies have found evidence that Alpha-Linolenic acid is related to a lower risk of cardiovascular disease. However, the mechanism is still unclear: The body converts Alpha-Linolenic acid into the longer chain fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and it is unknown whether the protective effect against cardiac arrhythmia is exerted by Alpha-Linolenic acid itself, or by these metabolic products. Some studies have linked Alpha-Linolenic acid with rapidly progressing prostate cancer and macular degeneration, increasing the risk 70% over control subjects (over those that did not receive Alpha-Linolenic acid). Research has also suggested a major neuroprotective effect of Alpha-Linolenic acid in in-vivo models of both global ischemia and KA-induced epilepsy. A large 2006 study found no association between total Alpha-Linolenic acid intake and overall risk of prostate cancer.

Dietary Alpha-Linolenic acid has been assessed for its role in cardiovascular health. Clinical benefits have been seen in some but not all studies. Still, a review in 2005 concluded "The weight of the evidence favors recommendations for modest dietary consumption of Alpha-Linolenic acid (2 to 3 g per day) for the primary and secondary prevention of CHD."