Walnut oil is a vegetable oil extracted from the kernel of the fruit of the walnut tree Juglans regia L. (the English or Persian walnut). The kernel contains 60-65% oil by weight, a very high proportion compared with most oilseeds. The oil is extracted by cold pressing only, to preserve its distinctive flavor and heat-sensitive fatty acids, and it is one of the most expensive vegetable oils on the market.

The walnut tree is native to Central Asia and Iran (ancient Persia), from where its cultivation spread along trade routes to Europe in the Roman era and to China during the Han dynasty. China is the world's largest producer of walnuts, followed by the United States (California in particular), then Iran, Chile, and Ukraine. France and Italy are leading producers of traditional walnut oil.

Walnut oil has a unique, balanced composition: 53% linoleic acid (omega-6), 13% linolenic acid (omega-3) — a high proportion compared with most nut oils — and 18% oleic acid. The omega-6:omega-3 ratio is about 4:1, an ideal health ratio and one of the best among vegetable oils. It contains good levels of vitamin E and strong antioxidants. However, its high omega-3 content makes it thermally unstable.

USDA-FAS data are not available for walnut oil production, as it is a specialty luxury oil. Global production is relatively small and concentrated in France (the traditional Dauphiné and Dordogne regions), Italy, and Chile. French oil in particular is known for its high quality and in many cases carries protected designation of origin labels. The oil is governed by Codex Alimentarius standard CXS 210-1999.

Walnut oil is used in food raw only (for salads, dressings, and cold pastries), because heating destroys its omega-3 and alters its flavor. It is added at the end of cooking for fine dishes and French and Italian salads. Industrially, it goes into luxury cosmetics and massage oils, and into the making of artists' paints and varnishes (for oil paintings and wood) as a "drying oil."