uglans regia, the Persian walnut, English walnut, Circassian walnut, or especially in Great Britain, common walnut, is an Old World walnut tree species native to the region stretching from the Balkans eastward to the Himalayas and southwest China. The largest forests are in Kyrgyzstan, where trees occur in extensive, nearly pure, walnut forests at 1,000–2,000 m (3000 to 7000 ft) altitude. It is widely cultivated across Europe.

Walnut trees grow best in rich, deep soil with full sun and long summers, such as the California central valley. In the U.S., J. regia is often grafted onto a rootstock of a native black walnut, Juglans hindsii to provide disease resistance.

Other plants often will not grow under walnut trees because the fallen leaves and husks contain juglone, a chemical which acts as a natural herbicide. Horses that eat walnut leaves may develop laminitis, a hoof ailment.