A walnut is the nut of any tree of the genus Juglans (Family Juglandaceae), particularly the Persian or English walnut, Juglans regia.

Technically a walnut is the seed of a drupe or drupaceous nut and thus not a true botanical nut. It is used for food after being processed, while green for pickled walnuts or after full ripening for its nutmeat. Nutmeat of the eastern black walnut from the Juglans nigra is less commercially available, as are butternut nutmeats from Juglans cinerea. The walnut is nutrient-dense with protein and essential fatty acids.