Linseed oil, also known as flaxseed oil or flax oil (in its edible form), is a colorless to yellowish oil obtained from the dried, ripened seeds of the flax plant (Linum usitatissimum). The oil is obtained by pressing, sometimes followed by solvent extraction.

Owing to its polymer-forming properties, linseed oil is often blended with combinations of other oils, resins, or solvents as an impregnator, drying oil finish, or varnish in wood finishing; as a pigment binder in oil paints; as a plasticizer and hardener in putty; and in the manufacture of linoleum. Linseed oil use has declined since the 1950s with increased availability of synthetic alkyd resins, which function similarly, are petroleum-based, and resist yellowing.

Linseed oil is a triglyceride, like other fats. Linseed oil is distinctive for its unusually large amount of α-linolenic acid, which oxidises in air. The fatty acids in a typical linseed oil are of the following types:

Having a high content of di- and tri-unsaturated esters, linseed oil is susceptible to polymerization reactions upon exposure to oxygen in air. This polymerization, which is called autoxidation, results in the rigidification of the material. To prevent premature drying, linseed oil-based products (oil paints, putty) are stored in airtight containers.

Rags soaked with linseed oil pose a fire hazard because they provide a large surface area for rapid oxidation. The oxidation of linseed oil is exothermic, which may lead to spontaneous combustion. In 1991, One Meridian Plaza, in Philadelphia, was severely damaged in a fire, in which three firefighters perished, thought to be caused by rags soaked with linseed oil.

Most applications of linseed oil exploit its drying properties, i.e., the initial material is liquid or at least pliable and the aged material is rigid but not brittle. The water-repelling (hydrophobic) nature of the resulting hydrocarbon-based material is advantageous.

Linseed oil is the carrier used in oil paint. It can also be used as a painting medium, making oil paints more fluid, transparent and glossy. It is available in varieties such as cold-pressed, alkali-refined, sun-bleached, sun-thickened, and polymerised (stand oil). The introduction of linseed oil was a significant advance in the technology of oil painting.^{[citation needed]}

Traditional glazing putty, consisting of a paste of chalk powder and linseed oil, is a sealant for glass windows that hardens within a few weeks of application and can then be painted over. The durability of putty is owed to the drying properties of linseed oil.^{[citation needed]}