Flaxseed oil is a vegetable oil extracted from the seeds of the flax plant Linum usitatissimum, and is one of the oldest oils known in human history. It is also known as "linseed oil" (for industrial uses) or "flaxseed oil" (for food uses). The seed contains 35-45% oil by weight.

Flax cultivation has a history extending more than eight thousand years in the Fertile Crescent and to Pharaonic Egypt, where linen garments were woven for mummies and the oil was used in embalming. After cotton fiber was industrially developed, flax became less used in textiles, and instead its oil production led to industrial roles (paints), then it recently returned to prominence as a healthy source of plant-based omega-3.

Flaxseed oil is very unique in its fatty composition: it contains an exceptional proportion of linolenic acid (omega-3) reaching 53%, and linoleic acid (omega-6) at 14%, with an omega-6:omega-3 ratio of 1:4 (the only one among vegetable oils in which omega-3 exceeds omega-6). The high omega-3 concentration makes it thermally unstable and unsuitable for cooking.

USDA-FAS data are not available for flax oil production, as it is a specialty oil, and Russia, Canada, Kazakhstan, and China lead global seed production. A large part of global production goes to industry (paints, insulation), and the food oil market is much smaller. The oil is governed by Codex Alimentarius standard CXS 210-1999.

Flaxseed oil is used in food cold (in salads, smoothies, and yogurt), and not for cooking, to avoid destroying omega-3 with heat. Industrially, the oil is considered the best "drying oil" in the world, and goes into the making of oil paints and varnishes, linoleum flooring paste, leather dyeing, and the treatment, finishing, and preservation of wood.