Quercus dumosa, commonly known as Nuttall's scrub oak or coastal sage scrub oak, is a species of scrub oak belonging to the white oak group (subgenus Quercus, section Quercus). It is a shrub typically reaching heights of 1–3 m (3.3–9.8 ft), characterized by a scraggly and rounded appearance, with dense tangled reddish branches, small dark green leaves with distinct teeth, curly trichomes on the lower leaf surfaces, and narrow acute acorns.

Native to the United States and Mexico, it is a rare species found in chaparral and coastal sage scrub on sandy soils often within sight of the ocean, and is found in disjunct populations ranging from Santa Barbara County, California south to the vicinity of Punta Colonet, Baja California. It is perhaps one of the rarest and most threatened species of oak in California, as the human development of coastal southern California has left very little habitat intact.

Quercus dumosa is an evergreen shrub typically growing 1–3 m (3.3–9.8 ft) tall, with some plants up to 5 m (16 ft) tall observed in the canyons of San Diego County, and others occasionally taking the form of low, matted clumps. It has a rounded and scraggly appearance, with tangled, slender reddish or grayish twigs and branches. The branches are often sharply angled. The bark is smooth when young, but eventually becomes scaly. The dark-green leaves are brittle and leathery, and fall off when disturbed. The leaves have a short petiole 1–2 mm (0.039–0.079 in) long. The leaf blade is usually 10–20 mm (0.39–0.79 in) long and has an undulate or cupped appearance, with a cordate base. The leaf margins may be abruptly pointed with spines or teeth or shallowly lobed, and rarely entire (smooth).

One of the most distinctive characters of Quercus dumosa are the trichomes (hairs) most prominent on its lower leaf surfaces. It has erect, twisted (curly), multi-rayed trichomes up to 0.5 mm (0.020 in) long, a feature unlike any other scrub oaks. The trichomes at least require a dissecting scope or 10 power hand lens to clearly discern, but compared to other scrub oaks these trichomes are clearly visible to the naked eye.

The male flowers are minute catkins that bloom from February through April.

The fruit is a narrow acute acorn up to 1.5 centimetres (5⁄8 in) wide. Some individuals produce large crops of acorns, and some produce very few fruits. The acorns are dispersed by gravity as they fall from the tree, and by animals that pick them up, such as squirrels and jays. Animals eat them immediately or cache them for later. The acorns tend to germinate easily. Reproduction via seed generally occurs only in very moist years.

Quercus dumosa was originally described in 1842 by Thomas Nuttall from a type specimen collected in the "hills near Santa Barbara". For about 140 years after Nuttall's description, the concept of Quercus dumosa broadened to encompass nearly all of the scrub oaks in the white oak group of California and Baja California. Traditional taxonomies heavily relied on the leaf form as the distinguishing feature. As most of the California scrub white oak species have a similar leaf form, this resulted in a very broad concept of Q. dumosa (Q. dumosa sensu lato) that neglected other characters like acorn morphology and leaf vestiture.

The species concept of Q. dumosa has narrowed as taxonomic developments changed focus to patterns of variation in the trichomes, acorns, and habitats of white scrub oaks.