Notholithocarpus densiflorus, commonly known as the tanoak or tanbark-oak, is a broadleaf tree in the family Fagaceae, and the type species of the genus Notholithocarpus. It is a hardwood tree that is native to the far western United States, particularly Oregon and California. It ranges from 15–40 meters (49–131 feet) in height, with a trunk diameter of 60–190 centimeters (24–75 inches). There are a number of radical and incompatible perceptions of tanoak, it has been seen as a cash crop to treasured food plant to trash tree.

It can reach 40 meters (130 feet) tall in the California Coast Ranges, though 15–25 m (49–82 ft) is more usual, and can have a trunk diameter of 60–190 centimeters (24–75 inches). The bark is fissured, and ranges from gray to brown. The tree's average age appears to be 180 years, although some estimates reach as high as 300 to 400 years old.

The leaves are alternate, 8–13 cm (3–5 in), with toothed margins and a hard, leathery texture. At first they are covered in dense orange-brown scurfy hairs on both sides, which wear off over time, more slowly on the underside of the leaf. The leaves will persist for three to four years.

Flowers are unisexual, as is typical for members of the Beech Family (Fagaceae). The tree can flower during any season except winter, but typically blossoms appear in June, July, or August with coastal and low-elevation trees blooming the earliest. The small, solitary female flowers aggregate as the base of the erect male catkin, each subtended by a small bract. The flowers are wind or insect pollinated.

The seed is an acorn 2–3.5 cm (3⁄4–1+1⁄2 in) long and 2 cm in diameter, very similar to an oak acorn, but with a very hard, woody nut shell more like a hazel nut. The nut sits in a cup during its 18-month maturation; the outside surface of the cup is rough with short spines. The nuts are produced in clusters of a few together on a single stem. Tanoak acorns have distinguishing caps that are hairy, rather than scaly.

Currently, the largest known tanoak specimen is on private timberland near the town of Ophir, Oregon. It has a circumference of 7.9 m (26 ft), is about 2.51 m (8 ft 3 in) in diameter at breast height, and is 37 m (121 ft) tall with an average crown spread of 17 m (56 ft).

Members of populations in interior California (in the northern Sierra Nevada) and the Klamath Mountains in southwest Oregon are smaller, rarely exceeding 3 m (9 ft 10 in) in height and often shrubby, with smaller leaves, 4–7 cm (1+1⁄2–2+3⁄4 in) long; these are separated as "dwarf tanoak", Notholithocarpus densiflorus var. echinoides. The variety intergrades with the type in northwest California and southwest Oregon. Tanoak grows as a shrub on serpentine soils. This mutant is used in horticulture, due in part to its rarity.

Chrysolepis chrysophylla is similar, but the leaves are scaly underneath and the fruits are spiny.